Matcher: `matchSimply` should call `matchWrapper`

I don't understand why it does its own thing. That it definitely does shows in lacking long-distance information for the coverage checker:

True = True

should not warn (or at least could), but it sure does:

test3.hs:8:1: warning: [-Wincomplete-uni-patterns]
    Pattern match(es) are non-exhaustive
    In a pattern binding: Patterns not matched: False
  |
8 | True = True
  | ^^^^^^^^^^^

For a different example, consider this beauty:

data SBool (b :: Bool) where
  STrue :: SBool True
  SFalse :: SBool False

STrue = SFalse

warns with

test3.hs:14:1: warning: [-Woverlapping-patterns]
    Pattern match has inaccessible right hand side
    In a pattern binding: STrue = ...
   |
14 | STrue = SFalse
   | ^^^^^^^^^^^^^^

test3.hs:14:1: warning: [-Wincomplete-uni-patterns]
    Pattern match(es) are non-exhaustive
    In a pattern binding: Patterns not matched: SFalse
   |
14 | STrue = SFalse
   | ^^^^^^^^^^^^^^

Both inaccessible RHS and incomplete. But the first warning should actually be redundant (Edit: Actually, no. It certainly forces the matched thing, so inaccessible)! Yet the coverage can't see that, as matchSimply doesn't pass on the long-distance info for the RHS, so it's equivalent to seeing STrue = x.

This refactoring would also allow us to get rid of PmCheck.checkSingle, which is a curious function already. I believe it's only really doing something for PatBindRhs anyway. Seems like an awfully special case for no obvious reason.