gmp_scanf c99 hex floats

[Kevin Ryde]

I wrote:
>
> 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.

I changed my mind about the C++ part of this.  g++ libstdc++ doesn't
accept c99 hex floats, so I propose to follow that, ensuring the same
things will be parsed and not parsed.

On that same aim of parsing or not parsing the same, I think I'll use
the istream locale isspace test, and remove various non-standard
places whitespace is accepted, and remove "@" for a float exponent
separator.