For my app, I'd like to define the following property on my data context class:

public IMongoCollection<BsonDocument> C => data.GetCollection<RawBsonDocument>( "c" );

My purpose is to have a single IMongoCollection instance from which I can retrieve raw documents, but into which I can also insert non-raw ones.

This improvement involves adding `out` to IMongoCollection's definition:

public interface IMongoCollection<out TDocument>