Async operations

[smikhalevski]

• Renamed use to addOperation and added addAsyncOperation, so now async operations can be added to any shape. For example, this allowed to enable support of async shapes in ArrayShape.includes check:

const shape1 = d.string().checkAsync(async () => …);

const shape2 = d.array().includes(shape1);

shape2.isAsync;
// ⮕ true

• Added checkAsync, refineAsync, and alterAsync;

• Removed async to sync auto downgrade optimization, so if parseAsync is called on a sync shape it would use the async path instead of wrapping a sync path in a promise.

New semantics

_applyOperations now returns a promise if there are async operations.

If the shape overrides only _apply and doesn't override _applyAsync then it's only safe
to call this method as the last statement in _apply. Otherwise, it may return an unexpected promise.

If the shape overrides both _apply and _applyAsync then this method would always
synchronously return a Result inside _apply.