Implemented Standard C library uchar.h functions.

Added implementations of c16rtomb, c32rtomb, mbrtoc16, and mbrtoc32.
Also added a wcsndup function that takes an allocator* argument to
round out cwchar.hpp with it's strndup counterpart. Finally, some
bugs introduced with the UTF-8 error recovery logic in the last
update were fixed. As a result, some additional speed improvements
were head in the s-expression lexer/parser.