minor: minor raw BSON perf and API improvements

[patrickfreed]

This PR implements a few minor performance and API improvements to the raw BSON API that I noticed while benchmarking it / using it in the driver.

• RawDocumentBuf::append accepts and impl AsRef<str> for the key instead of an impl Into<String>
  • this allows us to avoid an allocation when we have string reference types
  • we can't really move a string into our buffer anyways, so for owned strings there isn't any perf penalty here
  • this also applies to the FromIterator implementation for RawDocumentBuf
• exposed all fields of RawJavaScriptCodeWithScopeRef and RawRegexRef
  • this is in line with other similar types
  • the getters were removed as part of this