Use `wmemchr` in stringlib if `sizeof(STRINGLIB_CHAR) == sizeof(wchar_t)`

[goldsteinn]

Feature or enhancement

(A clear and concise description of your proposal.)

memchr is currently used if sizeof(STRINGLIB_CHAR) == sizeof(char) because it faster
than the standard C loops alternatives.

wmemchr has often roughly the same performance as memchr and can be used
in the same way as memchr to speedup some of the string functions STRINGLIB(find_char)
and STRINGLIB(replace_1char_inplace) are the two easiest candidates.

Pitch

(Explain why this feature or enhancement should be implemented and how it would be used.
Add examples, if applicable.)

It would, in some instances, make wide_str.find(wide_str_of_len_one) faster.

Previous discussion

The idea was discussed a bit in #69009 but wasn't the main topic of the issue
or put into any patches.

[corona10]

Looks like @methane san will have interests in this issue :)

[goldsteinn]

@corona10 have a PR for it at 93034