Support if-let guards in matches macros by using :guard matcher

[SpriteOvO]

Tracking issue #153104.

This PR replaces the original if $guard:expr with new matcher $guard:guard in macros matches! and assert_matches! to support the use of if-let guards syntax in these macros. This syntax is already stable in #141295 for match arms.

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.

cc @programmerjake
r? libs

[SpriteOvO]

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.

[nia-e]

Hi - could you provide us some examples of where this might be a breaking change? We discussed this in the libs-api meeting but it wasn't quite clear what this might affect