[v3] add_unsigned/remove_unsigned tweaks

[Benjamin Kosnik]

Adds in signed char bits as discussed by Paolo and Howard, and tests.

In addition, this adds compile-time warnings on things like

__gnu_cxx::__remove_unsigned<float>::__type
__gnu_cxx::__add_unsigned<bool>::__type

Even though these constructs are admittedly bizarre, it looks like this "trait" is
converging on semantic meaning, and signed-ness means nothing for
floating point types. (Last time I checked.)

tested x86/linux

-benjamin
-------------- next part --------------
An embedded and charset-unspecified text was scrubbed...
Name: p.20060929-2
URL: <http://gcc.gnu.org/pipermail/libstdc++/attachments/20060929/376763d3/attachment.ksh>