Constant expression parser is a disgusting mess

[davidgiven]

It's a cut-and-pasted lobotimised crippled version of the main expression parser, full of bugs, and shouldn't really exist. Constants should be parsed as part of the main expression parser.

Doing this is more complicated than it sounds as sometimes we need to parse constants while parsing a non-constant, so the two need to nest sensibly.

This would probably also give us constant folding in the parser, rather than leaving it for the typechecker (which would simplify the typechecker).