API: First-class modules for components

[bryphe]

Issue: Currently, we have no way to distinguish whether a component came from the same 'component class' today. This is important when determining whether to reuse state/effects. #26 went a step forward but the API is a bit clunky:

let componentWrappingB = (~children, ~x, ()) =>
  TestReact.component(() => <bComponent />, 
  ~uniqueId="componentWrappingB",
  ~children);

I'm proposing moving to a new API for creating components, using first-class modules - a function that returns a module.

This allows us to do the following:

• Implicitly create a unique class id, instead of leaving that burden on the user
• Implement custom components in an 'uppercase' convention, which seems to be a dominant style in the React ecosystem.

The new syntax would be:
module ComponentWrappingB =
(val component((render, ~x, ~children, ()) =>
render(() => {

}, ~children);
));

The component function creates the module (val unpacks it), and there would be a render function passed to the component.

And could be used as <ComponentWrappingB x />.

The anatomy of the function would be:

module ComponentWrappingB = 
    (val component((render, /* ...props */, ~children, ()) => 
        render(() => {
            /* ...any hooks here... */

            /* ...return another custom component / primitive component */
            <aComponent testVal=x/>
        }, ~children);
));

I think this is a step in a better direction, but I'd be interested to see if there is a way we could scrap that render function. A potential point of confusion I could see is perhaps users putting hooks outside the render block - we should make sure that throws.