Allow type parameters on multi effects

[b-studios]

Currently only effect operations can take type parameters:

effect Foo {
  def op[A](...): ...
}

To express a state effect with operations get and put, we would like to also support type parameters on effects

effect State[S] { ... }

[b-studios]

This requires implementing unification of effect types in trait Substitution. Two effects should only be considered equal, if they are applied to the same type parameters.

[b-studios]

Addressed in #54