Provide a cleaner way to read fields in nested documents

Currently, if you want to access a field in a nested document, you must do something like

let a = doc["a"]?.documentValue?["b"]

It would be nice if users could instead do something like doc["a"]["b"].

However, this is not possible right now, as the subscript getter returns a BSON, and BSON objects are not subscriptable.

We could consider making BSON subscriptable. However, it's not clear what the behavior should be when a user tries to subscript something that isn't actually a document, for example, a String. 

If we do implement this via subscript, analogous changes should be made to the dynamicMemberLookup behavior, to allow users to do doc.a.b etc.