Implemented the following methods on PathBuf which
forward to the underlying OsString.

- capacity
- with_capacity
- clear
- reserve
- reserve_exact
- shrink_to_fit
- shrink_to

- Fixed incorrect `mut` usage
- Fixed style in accordance with tidy
- Marked all methods as unstable
- Changed feature identifier to path_buf_alias_os_string_methods

Feature gate changed to `path_buf_capacity` as per advice from @Mark-Simulacrum