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.