Proposed refactoring: parameterize Match over Pat

The data type Match in compiler/Language/Haskell/Syntax/Expr.hs is defined as follows:

data Match p body
  = Match {
        m_ext :: XCMatch p body,
        m_ctxt :: HsMatchContext p,
          -- See Note [m_ctxt in Match]
        m_pats :: [LPat p], -- The patterns
        m_grhss :: (GRHSs p body)
  }
  | XMatch !(XXMatch p body)

There's the body parameter that can be instantiated to LHsExpr or LHsCmd, while the patterns are hardcoded to use LPat. As it turned out in !11109 (comment 521920), we could also make use of parameterization over pat:

-         m_pats :: [LPat p], -- The patterns
+         m_pats :: [pat],    -- The patterns

An attempt to make this change revealed that it has enough knock-on effects that I suggest we do it as a separate refactoring, so that !11109 (closed) can focus on actually adding support for @-binders.