Add a field in SimplMode to turn off built-in rules

Motivation

Currently, built-in rules simply cannot be turned off during simplification:

-- GHC.Core.Rules
matchRule opts rule_env _is_active fn args _rough_args
          (BuiltinRule { ru_try = match_fn })
-- Built-in rules can't be switched off, it seems
  = case match_fn opts rule_env fn args of
        Nothing   -> Nothing
        Just expr -> Just expr

This poses a problem for what we are trying to do: we are developing a Core plugin that converts certain CoreExprs of type a -> b into CoreExprs of type M a -> M b for some M. Our plugin needs to call simplExpr on various parts of the expression, for purposes like inlining, case-of-case transformation, beta reduction etc. The one thing we do not want simplExpr to do is specialization - it is much easier and more efficient to directly convert == $fEqFoo ... than $fEqFoo5 .... The latter (1) has a strange name (2) has a different name for each Foo (3) may contain primops which are difficult to deal with.

I don't think our use case is the only one that would benefit from a simplExpr that allows turning off built-in rules. This would allow one to precisely control its behavior, e.g., only performing case-of-case and inlining, but nothing else, which seems useful in general.

Proposal

I think this can be easily achieved by adding a new field to SimplMode, say sm_builtin_rules, and then modifying the above code snippet to check this field. Its value would normally be True, but can be set to False if one knows what they are doing.