So it turns out that I can't hold `&str` in token because it makes it
impossible to encapsulate a source file in the larger context- self
referential structure problems again. Everything gets rebuilt so that
the source can be passed through. While we're at it, more things
become Rc<> because, man..... life it too short.
Semantics in particular has become a giant hub of the module state: we
can basically just hold an Rc<Semantics> and have everything we could
possibly want to know about a source file, computed lazily if
necessary.
