lang.yaml: Restore original load() semantics and introduce load_dict()
lang.yaml: Restore original load() semantics and introduce load_dict()

The change load() to return (key, value) pairs based on document
metadata broke standard pyyaml semantics.  Restore the old semantics and
introduce `load_dict()` method to return attribute dictionary from a
document.

Also, make it possible for document schemas to notify the constructor
that top-level document dict should be used as attribute dict.

edgedb/edgedb,edgedb/edgedb,edgedb/edgedb