Please note that this PR directly applies :guard to macros that are already stable, so the support for if-let guards in these macros is instantly stable.

When using these macros that accept :guard, users will not be required to use nightly and enable #![feature(macro_guard_matcher)]. However, when defining macros with :guard, nightly and #![feature(macro_guard_matcher)] are still required. Therefore, I think a Crater run is needed.

Alternatively, to be more safe, we only provide the :guard version of macros when users activate macro_guard_matcher, otherwise falling back to the original if $e:expr version. But I couldn't find a way to write something like #[cfg(unstable_feature = "macro_guard_matcher")] in stdlib.