Add Dict.foldlr simplifications and make existing Dict functions with extra arg generic

[lue-bird]

Dict simplifications should now be on par with Set (except for insert)

-- same for foldr
Dict.foldl f initial Dict.empty
--> initial

Dict.foldl (\_ soFar -> soFar) initial dict
--> initial

Bonus: Convert existing manual Dict operation checks that exist because the function takes 2 args to more generic helpers.