RawOsString API changed in os_str_bytes 7.0.0 and bat no longer builds
with the lessopen feature because:
- ::from_string() is deprecated and says to use new().
The documentation says that new(), like from_string() no longer
needs to copy the string.
- ::assert_from_raw_vec() is no longer merely deprecated and now
requires selecting the "conversions" feature of os_str_bytes.
Replaces PR#2938
