gmp_scanf c99 hex floats

[Kevin Ryde]

By way of warning, though it should be uncontroversial, I'm going to
add support for c99 style hex float parsing to gmp_scanf, like the
normal scanf does.

I'll do the mpf istream input similarly if I'm smart enough, and
unless anyone else beats me too it.

For some reason I'd got it into my head that that hex was a gnu
extension, which it's not, or at least not since c99 was finalized
it's not.  Still, better late than never.

I don't see any need for mpf_set_str and mpf_inp_str to do the same,
they have their own arrangements for hex etc.