Our raw BSON serializer currently does not parse extended JSON into BSON. For example, the following code will produce BSON bytes that store a document with a nested document containing a "$regularExpression" key rather than a document with a regular expression value:

let doc = doc! { "regex": { "$regularExpression": { "pattern": "a", "options": "b" } } };
let bytes = bson::to_vec(&regex).unwrap();

This is inconsistent with the behavior of the non-raw BSON serializer, which does parse extended JSON. We should update the raw serializer to be consistent with the non-raw serializer.