Modernize Unicode

C++11 types char16_t and char32_t are designed for overloading by Unicode encoding style; they are productively quite useful in Spirit.  So convert to using them as the fundamental types for UTF-16 and UTF-32.

Note that ischar() for 8-bit and 16-bit values just detects whether the value will fit into 8/16 bits, so I changed the one for UTF-32 to do the same; so it is always true.  As it was, you'll end up checking every character for <= 0x10FFFF before redundantly testing it for e.g. equality with 'x'.  Is this change Not Good for some reason?

Also fix high-bit characters being possibly sign-extended before being passed to std::isxxx() which is Very Bad.
Also remove names from unused parameters, so we can use -Wunused to find the really unused ones.
Also a few other minor things.
Yes, as the comment says: https://github.com/chipdude/spirit_x3/commit/6a2d397cef17eb289e74cab28a32dcdd63ca297c