remove isInitial check on recompute

[Rich-Harris]

Occurred to me that we can remove the isInitial check on computations by creating a changed object that includes all keys that computations depend on for the first run:

// instead of this...
App.prototype._recompute = function _recompute(changed, state, oldState, isInitial) {
  if (isInitial || changed.a || changed.b) {
    if (differs((state.c = template.computed.c(state.a, state.b)), oldState.c)) changed.c = true;
  }
}

// ...this:
App.prototype._recompute = function _recompute(changed, state) {
  if (changed.a || changed.b) {
    if (differs(state.c, (state.c = template.computed.c(state.a, state.b)))) changed.c = true;
  }
}