[Patch] First step towards fast parsing of float types

[Paolo Carlini]

Martin Sebor wrote:

> FWIW, your (i.e., libstdc++) handling of thousands_sep and
> decimal_point is off in one other respect as well. Stage 2
> specifies that these special characters be processed _prior_
> to any digits. Libstdc++ process them _after_. A super-
> pedantic test case is below ;-)

Thank you *very* much, for pointing this out and for the neat
testcase! Luckily seems not too difficult fix. Will work on
this issue too...

Thanks again,
Paolo.