Decode trait and derive macro for it

Relates to #96

Summary

• Decode impls for std types
• Derive Decode proc-macro
• Checks for actual msgpack structure in tests

Note

Encode and Decode traits will no longer be compatible with rmp_serde (they might accidentally be, but it won't be checked)

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