test: test that our ffi struct representations match tarantool ones

Summary

• chore: minor tweaks

• test: check our datetime msgpack encoding is exactly the same as tarantool's one

• fix: encode datetime values compactly the same way tarantool does it

• test: test that our ffi struct representations match tarantool ones

affected structs:

• decimal
• datetime
• uuid

Ensure that

• New code is covered by tests
• API is documented
• Changelog is up to date
• Version is bumped in the appropriate Cargo.toml files