Make ServerPart a type constructor synonym

Currently it is:
~~~~
type Happs.ServerPart a = Happs.ServerPartT IO a
~~~~
Could we instead have:
~~~~
type Happs.ServerPart = Happs.ServerPartT IO
~~~~
This way, I could use `Happs.ServerPart` in a monad transformers stack, e.g. `ExceptionalT Happs.Response Happs.ServerPart a`.
