Constraints on variables

[grncdr]

It seems that currently the grammar only supports type constraints in the case of module declarations. It would be very useful to be able to constrain type variables, e.g. to require a particular interface.

For example:

interface _Comparable
  def <=>: (untyped other) -> Integer | nil
end

class Heap[Elem : _Comparable]
  def add: (Elem) -> void
end