Merge Tuple and TupleBuffer

Summary

Merges Tuple and TupleBuffer into one structure Tuple. Old Tuple remains as BoxTuple but it's use is discouraged.

There are following reasons for this merge:

1. Distinction between tarantool tuple pointer and buffer seems excessive for the end user most of the time (as we discussed with @gmoshkin )
2. As we are going to try and switch all the space, index and netbox interfaces to accept and return raw Tuple, it's impossible to design these interfaces when Tuple is not a single structure

Unblocks #195

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