Define a new MIME type model, parser, and serializer

This addresses all open inline issues with respect to the parser and serializer, aligns both closer with implementations, except where those stood in the way of an improved model.

This also updates all of it to make extensive use of the Infra Standard.

See #42 for the testing story (included all linked issues) and web-platform-tests/wpt#7764 for the majority of tests.

Author: annevk