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.