Renaming IS_AGGR_TYPE & co

[Paolo Carlini]

Hi all, hi Mark,

I'd like to finally work on this simple project but, before starting on
it, I need some guidance on related (re)names. Indeed, I noticed that we
have got, closely related:

    IS_AGGR_TYPE
    SET_IS_AGGR_TYPE
    IS_AGGR_TYPE_CODE
    PROMOTES_TO_AGGR_TYPE
    make_aggr_type (in lex.c)

In my understanding we should rename all of them, consistently, but I'm
not sure about the names, besides MAYBE_CLASS_TYPE_P for the first and,
likely, SET_MAYBE_CLASS_TYPE_P for the second.

Suggestions?

Many thanks,
Paolo.