Rollup merge of #137777 - thaliaarchi:os_string-push-str, r=joboet

Specialize `OsString::push` and `OsString as From` for UTF-8

When concatenating two WTF-8 strings, surrogate pairs at the boundaries need to be joined. However, since UTF-8 strings cannot contain surrogate halves, this check can be skipped when one string is UTF-8. Specialize `OsString::push` to use a more efficient concatenation in this case.

The WTF-8 version of `OsString` tracks whether it is known to be valid UTF-8 with its `is_known_utf8` field. Specialize `From<AsRef<OsStr>>` so this can be set for UTF-8 string types.

Unfortunately, a specialization for `T: AsRef<str>` conflicts with `T: AsRef<OsStr>`, so stamp out string types with a macro.

r? ``@ChrisDenton``

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