Generate multiple toProtocol() for runtime polymorphism instead of pick

[glassesneo]

protocol Animal:
  proc eat

protocol CanFly:
  proc fly

class Cat impl Animal:
  proc eat =
    discard
  # automatically defined
  proc toProtocol(): Animal

class Bird impl (Animal, CanFly):
  proc eat =
    discard
  proc fly =
    discard
  # automatically defined
  proc toProtocol(): Animal
  proc toProtocol(): CanFly
  proc toProtocol(): Animal & CanFly

[glassesneo]

converter may be good for it

[glassesneo]

& operator in #140