Fail fast on pattern synonyms
We were recovering too eagerly from errors in pattern-synonym type inference, leading to a cascade of confusing follow up errors (Trac #15685, #15692). The underlying issue is that a pattern synonym should have a closed, fixed type, with no unification variables in it. But it wasn't! Fixing this made me change the interface to simplifyInfer slightly. Instead of /emitting/ a residual implication constraint, it now /returns/ it, so that the caller can decide what to do.
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