Match rmp_serde's deserialization performance

msgpack is a binary format similar to BSON, and rmp_serde is a crate providing serialization/deserialization functionality similar to our BSON library. rmp_serde outperforms bson when deserializing from a slice of bytes, however. We should strive to improve bson 's performance to meet that of rmp_serde.

https://docs.rs/rmp-serde/0.15.5/rmp_serde/

benchmarks: https://github.com/mongodb/bson-rust/issues/264#issuecomment-873531262