Add tests for the bson.Element type and implement the WriteTo method.

Consolidate this type into ReaderElement and rename ReaderElement to Element.

For methods that return a document, add two methods, one that returns a Reader and another that returns a Document. Determine if getting a Document from a Reader's Element should panic or return a mutable Document from the underlying bytes.

Switch calls to binary.LittleEndian.Uint32 to use the readi32 method.