Roll custom versions of isalpha/isalnum.
Roll custom versions of isalpha/isalnum.

Avoids locale issues and avoids unexpected assertions for non-ASCII
values.

KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross