Do not hardcode arguments to FunTy

The function type takes 5 arguments (multiplicity, two RuntimeReps, argument, result).

In the compiler, there are many places where we pattern match on lists of length 5, or hardcode particular indices. For example:

decomposeFunCo r co = ASSERT2( all_ok, ppr co )
                      (mkNthCo Nominal 0 co, mkNthCo r 3 co, mkNthCo r 4 co)

...
    go r n co@(FunCo r0 w arg res)
      -- See Note [Function coercions]
      -- If FunCo _ mult arg_co res_co ::   (s1:TYPE sk1 :mult-> s2:TYPE sk2)
      --                                  ~ (t1:TYPE tk1 :mult-> t2:TYPE tk2)
      -- Then we want to behave as if co was
      --    TyConAppCo mult argk_co resk_co arg_co res_co
      -- where
      --    argk_co :: sk1 ~ tk1  =  mkNthCo 0 (mkKindCo arg_co)
      --    resk_co :: sk2 ~ tk2  =  mkNthCo 0 (mkKindCo res_co)
      --                             i.e. mkRuntimeRepCo
      = case n of
          0 -> ASSERT( r == Nominal ) w
          1 -> ASSERT( r == Nominal ) mkRuntimeRepCo arg
          2 -> ASSERT( r == Nominal ) mkRuntimeRepCo res
          3 -> ASSERT( r == r0 )      arg
          4 -> ASSERT( r == r0 )      res
          _ -> pprPanic "mkNthCo(FunCo)" (ppr n $$ ppr co)

During development of linear types, we added a new argument and it was hard to find all occurrences.

We should use named constants whenever possible. Instead of lists, we could have pattern synonyms tweag/ghc@0a678646. Last time I've checked though, pattern synonyms caused performance regressions.