Allow as-patterns in unidirectional patttern synonyms
This patch implements GHC Proposal #94, described here https://github.com/ghc-proposals/ghc-proposals/pull/94 The effect is simply to lift a totally-undocumented restriction to unidirecional pattern synonyms, namely that they can't have as-patterns or n+k patterns. The fix is easy: just remove the checks. I also took the opportunity to improve the manual entry for the semantics of pattern matching for pattern synonyms.
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