Tracking issue for CString, OsString, PathBuf extra methods

[clarfonthey]

This includes the methods:

• CString::into_boxed_c_str
• OsString::into_boxed_os_str
• PathBuf::into_boxed_path
• <Box<CStr>>::into_c_string
• <Box<OsStr>>::into_os_string
• <Box<Path>>::into_path_buf
• CString::as_c_str

The into_boxed_* methods coming from #39594, the other into_* methods from #40009, and the as_c_str method from #41095.

The features for the methods are into_boxed_c_str, into_boxed_os_str, into_boxed_path, and as_c_str.

[clarfonthey]

ping @brson / @alexcrichton to tag this, also ping @jonhoo because you were specifically interested in stabilising these for something for serde.

[alexcrichton]

Oh it looks like in #39594 we forgot to update the tracking issue. @clarcharr could you update those tracking issues to point here?

[clarfonthey]

I included that as part of the second PR, so, already covered. If for some reason that PR isn't accepted, I'll make a separate PR for just updating the tracking issues.