New declaration syntax: arguments.

This patch introduces the new declaration syntax, for function arguments only. parse_decl() is the new workhorse, and it factors out a lot of custom logic that is hardcoded and duplicated into many areas of sc1.c. Unfortunately, it's a very complicated function: the grammar has lots of ambiguities and it has to do deep pattern matching to figure out which type of declaration to parse as.

nextmap.sp is going to be my testing ground for the new syntax, so here I've ported all its methods to use new-style declarations in its arguments.

Eventually I would like to keyword "float", "bool", and "char". I could probably get away with it. But I'll save that for later.