isolated some problematic macros from z80.h
as the macros _C _A _B _L are defined by some compilers in ctype.h
compilation will fail, when z80.h and string.h or ctype.h are referenced
by the same c file. thus this refactoring.
Thanks for this contribution.
I like it, it's a much cleaner separation between z80 and its internals.
I'd like to integrate it once the two comments are addressed.