Hide other unmatchable patterns

Lambdas, in particular. Fixes #1671
Fix checkPossible when unifying bound vars

It's not a rigid fail if it's two bound variables that differ.
Fixes #1108
Coverage checker should ignore hidden arguments

Since they're required to be inferred from other arguments, they make no
contribution to coverage checking.
Fixes #1710
Undo dubious optimisation in resolveTC

Since hnf_compute doesn't do what it says, and it's causing some obscure
or complicated type class constraints not to resolve properly.
Fixes #1688
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