[Patch] Some work on money_get::do_get

[Benjamin Kosnik]

>that I didn't like (repeated calls to widen, repetead calls to erase...).

Yep... it would be nice to have the all facets do work in one character
type, minimizing ctype calls, like numpunct/num_get/num_put post Jerry
Quinn.... I think that is on the TODO list. (Of course, I do need to
update that: I'll do that today.)

>In the process, I noticed two conformance issues (see testcases):
>
>  1- According to 22.2.6.1.2, p4, "... preceded by a minus sign if and
>     only if the result is negative" (see first test).
>  2- We tried to fail whenever __tmp_units was empty, but failed to do
>     it when _sign.size() && __sign == __neg_sign (see second test).
>
>I find the final result quite clean and efficient ;)

Agreed. Thanks for this!

-benjamin