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.
Unfortunately, a specialization for `T: AsRef<str>` conflicts with
`T: AsRef<OsStr>`, so stamp out string types with a macro.
a8d78fec52