It is sometimes needed to be able to parse the value component of a BSONElement memory buffer without having a pointer to the full BSONElement binary containing the type byte and field name.

We should have an abstraction for this use-case that can be re-used so we don't have to implement custom BSON parsing code throughout the code when this use-case comes up.

The main use-case is to be able to abstract code for BSONElement and BSONObj + type.