[WIP] Support C++11 <cuchar>

[Ed Smith-Rowland]

Greetings,

This effort tries to add the C++11 header <cuchar> that is a wrapper for the C11 header uchar.h.  This feature supports conversion to and from char16_t/char32_t and multibyte sequences and is supported on newer glibc systems.

Honestly, my main goal was to check off a low-priority box and to learn some things ahead of finishing, say, filesystem.  The main things that are getting me are configuration issues and autohell.  Also, in the cases where uchar.h is not supported - what to do?  I have a #warning in cuchar about missing support and the guts are then #if-d out.  I want a macro maybe to let the user test if this <cuchar> is actually working on their system.

Ideas and help is most appreciated.
Thanks,

Ed Smith-Rowland
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