Naming of "ireturn"

[lspitzner]

With the AMP update, ireturn is somewhat outdated. I propose a new naming scheme:

class IxPointed where ipoint :: ..

ipure :: IxApplicative m => a -> m i i a
ipure = ipoint

ireturn :: IxMonad m => a -> m i i a
ireturn = ipoint

This is of course a breaking change for any existing instances.