Re-introduce Placeholder System

[harrysolovay]

Ideally, we'll be able to apply to any effect (and have it scoped to that subtree).

const n_ = Symbol();
const n = Z._<number>(n_);

const squared = Z.call(n, (n) => {
  return n * n;
});

const applied = squared.apply(n, 2);

const result = applied.run();