Translate wchar.h -> crt/wchar.bi
Don't translate plain wchar_t to plain wstring

Plain wstring isn't allowed in FB. It should be translated to wchar_t (the
target-specific integer type).

I.e. wchar_t/wstring are now handled very much like char/zstring. Using
wchar_t requires us to #include crt/wchar.bi since it's not a built-in,
but that's ok.