Define the precise interface of a type, and how it interacts with the compiler.

There is the beginning of a very crude definition of what a type is. It needs to be fleshed out, and to interface correctly with both the interpreter and the compiler.

There will be some "meta" connections here, since type is itself a type, so I'm afraid the full definition cannot be given solely in XL without resorting to a builtin trick along the way.