Revamp the scheduler API

[Dave]

The current api works but tends to depend upon closure, which can be a performance bottleneck. Instead we can redefine a scheduler as:

type t = {
  schedule:
    'state .
    (('state => unit, 'state) => unit, 'state) => Disposable.t,

};

let immediate: t = {
  schedule: (work, state) => {
    let rec continuation = state => work(continuation, state);
    work(continuation, state);
    Disposable.disposed;
  },
};

And define a delay scheduler as:

type t = {
  schedule:
    'state .
    (~delay=float, ((~delay=float, 'state) => unit, 'state) => unit, 'state) => Disposable.t,
};

[Dave]

landed a big refactor. Still more tweaks to be made.