Add `create_foo()` static methods for the missing Token::Types, and use
them in the Tokenizer. This means we slightly deviate from the spec now:
it says "create foo token... set its bar to 32", but we now just wait
and construct the Token fully-formed. But those cases are short so it
should still be clear what we're doing.
This makes it possible to construct all kinds of Token elsewhere, such
as for testing purposes.
