Add suggestion quick fixes

E.g.

* Pointfree conversions
* Eta-reductions (`\x -> f x` to `f`)
* Const (`\_ -> x` to `const x`)
* Void (`x >> return ()` to `void x`)
* ...
* Common library functions (`maybe x id m` to `fromMaybe x m`, `fst . runState` to `evalState`, `snd . runState` to `execState`, `foldr1 (?)` to `anyOf`, ...)
* Redundant `do`s (`do { x }` to `x`)
* Merging adjacent `let`s
* Warn (or at least 'info') about `Debug.Trace` imports
* ...

These suggestions could be similar to what hlint suggests.