Parse namespaces for classes

Also for superclasses. Unexpectedly, this was easy to extend to multiple
namespaces.
Lowerclass class / module names are a parse error

Surprising! I suppose it's nice that this fails early, rather than waiting
until it gets to the end of a script to finally fail when it must interpret
a class description.
Parse module declarations
Parse assignment expressions

"foo = 5" isn't too hard to parse, it turns out.