The bsonrw.Copier type is an empty struct that exposes a collection of functions related to copying BSON or Extended JSON documents. Most of the functions are only used in the BSON library internally. Only the CopyValueToBytes function has known uses outside of the bson package (in the mgocompat package and in ADL code). It's possible that function can be replaced by other functionality already in the bson package. If that's not the case, add a package function called CopyValueToBytes that replaces Copier.CopyValueToBytes.

Definition of done:

• Deprecate the bsonrw.Copier type and all functions on that type.
• Deprecate the bsonrw.CopyDocument function.
• Investigate whether CopyValueToBytes can be replaced by bson.MarshalValue. If so, suggest using that function. If not, add a package function called CopyValueToBytes and suggest using that function instead.
• Investigate deprecating the bsonrw.BytesWriter and bsonrw.BytesReader interfaces, which may not be necessary if bsonrw.Copier is fully deprecated.