Tweak .merlin

* get rid of useless PRJ flag
* add lwt syntax extension flag
Cosmetic improvements

* reduce parens by using |>
* remove unused vars and unnecessary type annotations (present in the
  mli anyway)
* simplify let f x = match x with ... to let f = function
* remove unused vars
Merge pull request #199 from rgrinberg/master

Cosmetic improvements + .merlin