Waiting for a valid `leader_id` on a node isn't enough. It may already
have one, but still be a Learner. Instead, the fixture should wait until
the node is promoted to voter.

The assertion `status == "Leader"` was in the first place, and
`raft_timeout_now` call was unreachable.

1. Print logs to the stderr so that they interleave with tarantool logs.
2. Fix `cluster.__repr__()`.

1. Review `Pipfile`:
  - Remove unused `filelock`;
  - Install `mypy` - static type checker for Python.
2. Add new command `pipenv run lint`.
3. Enable `mypy` in CI. Fix reported errors in `test_basics.py`.
4. Renew readme.

In `conftest.py`:

  - *Add* function `xdist_worker_number`. It converts `str(worker_id)` into `int`.
    It serves as a substitute for `session_data_mutex` for parallel test runs.

  - *Change* `normalize_net_box_result`. Replace a function with a decorator. Also,
    handle typical kinds of picodata responses. Extensively test it in
    `test_basics.py::test_eval/call_normalization`.

  - *Add* dataclass `RaftStatus`. It shouldn't be used outside `conftest.py`. It only makes
    assertions more brief in logs and understndable in code.

  - *Add* all raft stuff into the `Instance` class. This implies `raft_propose_eval`,
    `assert_raft_status` all `promote_or_fail` moved from `util.py`.

  - *Change* fixture `compile`. No need in extra logics since commit 59c31cb8.

  - *Preserve* fixture `binary_path`.

  - *Remove* fixtures `session_data_mutex` and `run_id`. Superseded with `xdist_worker_number`.

  - *Remove* fixtures `run_cluster` and `run_instance`. Superseded with `cluster.deploy(...)`.

  - *Remove* function `wait_tcp_port`. It's never enough to check raw socket. Superseded with
    `instance.wait_ready()`.

  - Give the instances clean names `i1, i2, ...`, and simple addresses `127.7.0.1:3301`.
    For the parallel test run use different IPs `127.7.N.1` etc.

In `test_basics.py`:

  - *Add* `test_xdist_worker_number`.

  - *Add* `test_call_normalization` and `test_eval_normalization`.

  - *Add* `test_process_management`. It's brand new, never implemented in luatest before.

  - *Rename* `test_single_instance_raft_eval` to `test_propose_eval` and extend it with additional
    assertion from `single_test.lua`.

  - *Remove* `test_instance`. A part of its logics is moved to `test_call/eval_normalization`. The
    other part is rewritten and extended in `test_process_management`.

  - *Remove* `test_cluster`. It was completely useless because of inappropriate synchronization and
    no valuable assertions.

  - *Remove* `test_propose_eval`. It wasn't that useful, but failed because of inappropriate
    synchronization.

In `test_couple.py`:

  - *Preserve* `test_follower_proposal` and `test_failover`. Just slightly refactor according to the
    new `conftest.py` API.

In `util.py` (completely removed):

  - *Remove* decorator `retry`. Needless.

  - *Remove* decorator `retry_on_network_errors`. Inappropriate predicate didn't catch Lua errors.

  - *Remove* everything related to raft. Move it into `conftest.py`.

Part of https://git.picodata.io/picodata/picodata/picodata/-/issues/59

We've recently noticed many files in `vendor/` dir have windows-like EOL
`crlf`. When cloning the repo, git produces a huge diff due to the EOL
normalization built-in feature. This patch disbles it for the certain
path.

Close https://git.picodata.io/picodata/picodata/picodata/-/issues/56

We don't want a child process to live without the supervisor.

Usually, supervisor waits for child forever and retransmits termination
signals. But if the parent is killed with a SIGKILL there's no way to
pass anything.

This patch supplies a child process with a `supervisor_fuse` fiber. It
tries to read from a pipe (that supervisor never writes to), and if the
writing end is closed, it means the supervisor has terminated. In this
case, child process terminates too.

Part of https://git.picodata.io/picodata/picodata/picodata/-/issues/56

This patch reduces the variable passing trace length. Instead of
returning it from function to function, call `exit()` in place.

The trace of exit code before the patch:
```
exit() <- main <- main_run <- fork(child) <- tarantool_main
```

And now:
```
exit() <- tarantool_main
```

Type safety is ensured at comilation time by using `!` type for
`fn main()`. See https://doc.rust-lang.org/reference/types/never.html

```console
$ cargo clippy
   Compiling picodata v0.1.0 (/home/rosik/w/picodata)
warning: 'tarantool-sys/patches-applied' exists, so patching step is
skipped
warning: single-character string constant used as pattern
   --> src/args.rs:194:47
    |
194 |     let (host, port) = match text.rsplit_once(":") {
    |                                               ^^^ help: try using
a `char` instead: `':'`
    |
    = note: `#[warn(clippy::single_char_pattern)]` on by default
    = help: for further information visit
https://rust-lang.github.io/rust-clippy/master/index.html#single_char_pattern

warning: called `cloned().next()` on an `Iterator`. It may be more
efficient to call `next().cloned()` instead
   --> src/discovery.rs:154:27
    |
154 |                 let res =
    peers.difference(&self.visited).cloned().next();
    |
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this:
`peers.difference(&self.visited).next().cloned()`
    |
    = note: `#[warn(clippy::iter_overeager_cloned)]` on by default
    = help: for further information visit
https://rust-lang.github.io/rust-clippy/master/index.html#iter_overeager_cloned

warning: you seem to be trying to use `match` for destructuring a single
pattern. Consider using `if let`
  --> src/ipc.rs:27:9
   |
27 | /         match rmp_serde::encode::write(&mut self.fd, msg) {
28 | |             Err(e) => tlog!(Error, "ipc error: {e}"),
29 | |             Ok(()) => (),
30 | |         }
   | |_________^ help: try this: `if let Err(e) =
rmp_serde::encode::write(&mut self.fd, msg) { tlog!(Error, "ipc error:
{e}") }`
   |
   = note: `#[warn(clippy::single_match)]` on by default
   = help: for further information visit
https://rust-lang.github.io/rust-clippy/master/index.html#single_match

warning: `picodata` (bin "picodata") generated 3 warnings
    Finished dev [unoptimized + debuginfo] target(s) in 1.41s
```

1. Remove it from functions args where it's not necessary.
2. In other places rename it into `supervisor_ipc`.

Since dynamic discovery isn't implemented yet, instance after
`raft_join` has discovery uninitialized. It's not a reason to panic.

Use common `Notify` instead of a cond. It makes the code more clear and
reliable, because each request is tracked individually and can't be
affected by different yields in the main_loop.

Replace macros with function in `traft::node`.