These functions should be declared in `stdlib.h`, not `wchar.h`, as
confusing as it is. Move them to the proper header file and matching
directories in src/ and test/ trees.
This was discovered while testing libc++ build against llvm-libc, which
re-declares functions like mbtowc in std-namespace in `<cstdlib>`
header, and then uses those functions in its locale implementation.
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