Static variables consume memory and can be subject to less
optimization. This variable is only used in 1 place and can be moved
into the function and make it non-static.
This patch introduces CharacterTypes.h, which aims to be a replacement
for most usages of ctype.h. In contrast to that implementation, this
header makes use of exclusively constexpr functions and support the full
Unicode code point set without any narrowing-conversion issues.
