Implement multi effects

[b-studios]

Right now every effect operation corresponds to one effect. It is planned to relax this and allow multiple operations. For this we would change declaration syntax from:

effect Flip(): Boolean

to:

effect Amb {
  def flip(): Boolean
}

The old syntax could be syntactic sugar for:

effect Flip {
  def flip(): Boolean
}

Effect invocation could be

do Amb.flip()

where

def flip(): Boolean / Amb = do Amb.flip()

could automatically beingbegenerated, so typically effect operations would be invoked as:

flip()

[b-studios]

Closed by #13