1. Prefer using `"str".into()` since it's briefer.
2. Preserve `"str".to_owned()` if type inference can't help.

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

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

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/9