ast: Remove now unused parameter/result of astCheckConst()
ast: Use astNewUOP() to negate CONST node instead of doing it manually
ast: Turn astGetValueAsStr() into astConstFlushToStr() (see also 7c62ad2d)
ast: astGetValueAsWstr(): ditto
ast: Remove astCONST2FBValue(), use astGetValue() instead
Fix internal c/re/allocate() declarations to use unsigned size parameters

(they use size_t which is unsigned; this fixes type mismatch problems
with the C backend)