clarify decimal future, fix encoding

Summary

• chore: clarify standalone_decimal feature

• fix: use more efficient serialization with standalone decimal

Previously we used to use full size u32 to represent decimal scale on the wire. Message pack supports variable sized integers, so in case number is small enough it can be represented with smaller number of bytes. This was previously missed when we switched tarantool decimal impl to standalone one. Now since we returned back to tarantool builtin implementation this surfaced in #200 (closed). So the patch fixes the encoding. As I and @gmoshkin checked the change is backwards compatible.

Add a test for encoding to match specific byte layout so we wont miss similar issue in the future.

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