Bring up-to-date with std

After three years, std's OsStr and OsString got new methods. Moreover, some unstable methods were stabilized and now make sense to be ungated in this crate.

• OsString::shrink_to has been stabilized in 1.56;
• OsString::try_reserve has been added and is now stable;
• OsString::try_reserve_exact has been added and is now stable;
• OsStr::as_os_str_bytes has been added behind a feature flag;
• OsStr::from_os_str_bytes_unchecked has been added behind a feature flag;
• OsStr::eq_ignore_ascii_case has been stabilized in 1.53;
• OsStr::is_ascii has been stabilized in 1.53;
• OsStr::make_ascii_lowercase has been stabilized in 1.53;
• OsStr::make_ascii_uppercase has been stabilized in 1.53;
• OsStr::to_ascii_lowercase has been stabilized in 1.53;
• OsStr::to_ascii_uppercase has been stabilized in 1.53;
• impl<'a> Extend<&'a UnixStr> for UnixString;
• impl<'a> FromIterator<&'a UnixStr> for UnixString;
• impl<S: Borrow<UnixStr>> Join<&UnixStr> for [S];
• impl<'a> TryFrom<&'a UnixStr> for &'a str;
• impl<'a> Extend<Cow<'a, UnixStr>> for UnixString;
• impl Extend<UnixString> for UnixString;
• impl<'a> FromIterator<Cow<'a, UnixStr>> for UnixString;
• impl FromIterator<UnixString> for UnixString;
• impl Write for UnixString.