Optimize certain liftEq cases

[treeowl]

GHC sometimes needs to know if two IntMaps have the same keys. Once my patch goes in, it will check this using liftEq (\_ _ -> True). This is pretty good, but it would be even better if it were rewritten to a specialized sameKeys function. We should be able to do this with rewrite rules that "test" whether the function ignores its arguments and always returns true.

[meooow25]

For this we should just inline liftEq. It's a small function. GHC optimizations can do the rest.