Update Integer, phase 2

[sampersand]

This is the second phase for updating Integer (phase one is #2995).

Here, we finish off the rest of the methods, including the ones which required coerce. The _Coerce interface (which is on Numeric) actually slots quite nicely:

class Numeric
  interface _Coerce[O, Other, Self]
    def coerce: (O other) -> [Other, Self]
  end
end

class Integer < Numeric
  def +: (Integer other) -> Integer
       | [S, R] (Numeric::_Coerce[self, RBS::Ops::_Add[S, R], S] other) -> R
end

We don't even need separate cases for other numerics, as coerce works!