Many bounds are currently of the form `T: ?Sized + AsRef<OsStr>` where the
argument is `&T`, but the pattern elsewhere (primarily `std::fs`) has been to
remove the `?Sized` bound and take `T` instead (allowing usage with both
references and owned values). This commit generalizes the possible apis in
`std::env` from `&T` to `T` in this fashion.
The `split_paths` function remains the same as the return value borrows the
input value, so ta borrowed reference is required.