For ambiguous concepts (such as Entry, Message) use prefix notation instead of suffixes.

This patch eliminates raw raft node dereferencing and redundant use of
`borrow_mut()`.  Instead, `traft::Node` provides a safe API that
communicates with the raw node through the fiber channel.

Also, functions `node::new` and `node::start` are merged because keeping
them distinct makes no sense.

As we recently discovered, Tarantool has a peculiarity - when calling
stored procedures with `{language = "C"}` option, it disregards the
shared object (`.so` / `.dylib`) already loaded by Lua and makes the
second independent `dlopen`. As a result, Lua and C stored procedures
can't share state, because even static variables point to different
memory locations.

As a workaround, this patch provides the API for hiding the state inside
`lua_State`. Under the hood, the stash consumes a custom struct and
leaks the wrapping box. Inside Lua, it's represented by a userdata,
which the `access()` function provides access to.

Use publicly available `tarantool::global_lua` instead of the self-made
implementation.

Tarantool specificities make it difficult to set up usual rust tests.
Picolib is coupled with tarantool symbols and compiled as a dylib, so
it can't be linked to a stand-alone binary.

This patch introduces another way of testing.

All tests (called inner) are already built into dylib and exposed in
the tarantool console:

```
tarantool> picolib.test
---
- test_version: 'function: 0x41f32400'
  test_traft_storage_RaftEntryRow: 'function: 0x41f32550'
...
```

The new integration test `inner.rs` iterates over them and runs each
one in a separate tarantool instance.

See also: commit 6610e5cb

Sergey V authored 3 years ago and Yaroslav Dynnikov committed 3 years ago

Originally it was `picolib::traft::storage::Message`.
Now it's simply `picolib::Message`.

Move tarantool+slog related stuff to a separate module `tlog`.
Introduce the `tlog!` macro for easier logging.

The clap module we use for parsing command-line arguments allows
specifying `default_value`. Unfortunately, it makes impossible to tell
if an argument was actually passed. So it always overrides env var with
a default value, which is undesirable.

1. Change raft_state schema. Now it's more suitable for updating
   individual fields of the raft hard state.

   Close https://gitlab.com/picodata/picodata/picodata/-/issues/13

2. Persist commit and applied indices.

   Close https://gitlab.com/picodata/picodata/picodata/-/issues/14

3. Rearrange storage code. High cohesion, low coupling, as they
   teach us.

Close https://gitlab.com/picodata/picodata/picodata/-/issues/12

This patch implements the Storage trait without MemStorage from example.
All data is persisted it tarantool spaces.

Also, Rust modules are slightly reorganized.

Close https://gitlab.com/picodata/picodata/picodata/-/issues/11

Close https://gitlab.com/picodata/picodata/picodata/-/issues/10

Close https://gitlab.com/picodata/picodata/picodata/-/issues/9

- There's a fiber which ticks logical clock.
- There's a `raft_propose` API exposed in the Tarantool console.

Close https://gitlab.com/picodata/picodata/picodata/-/issues/7
Close https://gitlab.com/picodata/picodata/picodata/-/issues/8

It used to panic if the env variable wasn't set.
Also, this patch covers it with tests.

The control flow of picodata looks as following:

Picodata binary -> exec tarantool -l picodata -> libpicodata.so -> ...

Since the libpicodata is a shared lib loaded by tarantool, we can't pass
it arguments as usual. At first, tarantool parses them by itself and
rejects uknown arguments. Secondly, it spoils argv array.

To overcome these two issues, this patch introduces argparse on behalf
of the picodata binary, and puts parsed values into the environment
variables.

P.S. Thihs is how command line args look from within libpicodata:

```console
$ tarantool -l picodata /dev/null --opt1 foo --opt2 bar
Hello from rust lib
arg[0] tarantool
arg[1] /dev/null
arg[2] --opt1
arg[3] foo
arg[4] --opt2
arg[5] bar
arg[6] --opt2
arg[7] bar
```

This patch introduces the new cargo package - picodata.

The package consists of two parts:
- The `picodata` binary does some env magic and executes
  `tarantool -l picodata`.
- The `libpicodata` dylib is the starting point for further
  development.