Feature: Express single child components in the type system

[bryphe]

(Related to the nesting-reconcilers discussion in #30, and the compositional component discussion in revery: revery-ui/revery#489) Also inspired by @jordwalke 's exploration on statically typed React trees: https://github.com/reasonml/reason-react/tree/StaticReactExperiment/explorations

There have been cases that have come up where it'd be nice to make some constraints on the tree at compile-time:

• Single-child components - It'd be helpful to express, via the type system, that a component can only have a single child. This came up in the compositional-component discussion in Proposal: API - Move from 'style' to Compositional Components revery-ui/revery#489, as it'd be useful for items like <Transform />

Based on discussion with @wokalski , this seems doable with a PPX

• Constraints between parent <-> children - Another useful case would be to express that only certain components can be children of a specific parent. An example might be a FlexLayout widget, that can only take Row, Column as first-level children. Other examples could be a <Menu> that only allows <MenuItem /> as children, etc. Fixed in Defunctorize Brisk_reconciler #46.

I think this may be something that could be supported via nested reconcilers - @wokalski mentioned that perhaps we could model this as a type constraint where element('a) is generic, element(menuComponent) is specific to a menu, etc.

[wokalski]

@bryphe Single child components can only work sensibly with full blown static react. If you use any other approximation it still produces invalid states. i.e.

let customComponent = () => <> <one/> <two/> </>

<Transform>
  <customCompnent />
</Transform>

The second one will be solved with #30