Overloaded "local" do-blocks.

Inspired by Local Do GHC-proposal. Yet because we do desugaring in reader phase, we must have a bit more complicated setup.

The expressions like

ex2d :: IxStateT Identity Int String ()
ex2d = ixmonad.do
    _unused <- ixmodify show
    ixmodify reverse

are desugared into

ex2b :: IxStateT Identity Int String ()
ex2b =
    ixmonad @Bind (ixmodify show) $ \_unused ->
    ixmodify reverse

Allowing to locally overload what do desugars to.

The monad in this module is an example how to define a desugaring. We need to it this way, so the names are easily accessible in renamer phase. (I.e. constant, then transformation is pure, as we don't need to lookup them for each do-block).

Enabled with:

{-# OPTIONS -fplugin=Overloaded -fplugin-opt=Overloaded:Do #-}