Remove 'a from wordLang AST onwards

@myreen pointed out that the only use of 'a in the AST (from wordLang onwards) is for constants.

It should be possible to represent these with nums instead, and have the entire compiler be non-dependent on 'a.

However, this is a very expensive refactor (the semantics still needs to know 'a, and some compiler passes will need to change so that they depend on a word size config).