Make match_ignore_ascii_case! future-proof.

Change the usage syntax to require a comma on the last non-fallback arm, which is a [breaking-change].

The old definition has started to emit a warning in recent nightlies and is likely to be an error in future versions: https://github.com/rust-lang/rfcs/pull/1384

The new definition is recursive to resolve ambiguities. Unfortunately this makes error reporting terrible: https://github.com/rust-lang/rust/issues/31022

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