Schema Definition Language Parser

This is a parser for the ad-hoc type definition language included in the
spec. This should also serve as a de-facto spec for the language.
Clearly once this is stable we should include it in the actual spec as
well.

This should end up being useful in a few contexts, including
client-side types and also a DSL for producing the JS type definitions
which can be tedious to write, and a way to specify acceptance tests.
(This would be a clear next step for this project)