* Rename past `Component` type to `ComponentWithChildren`.
  Motivation: Most components don't actually support children.
  This causes a type error upon accidental passing of children /
  forces you to think about which components support children.
* Added second parameter to `ComponentWithChildren`
  to make it easier to give a custom type for `children`
  (but still defaults to useful `JSX.Element`).
* New `Component` type does not have automatic `children` property
  (like React's preferred `VoidFunctionalComponent`),
  offering another natural way to type `props.children`:
  `Component<{children: JSX.Element}>`.
* `props` argument in both `Component` and `ComponentWithChildren`
  automatically cast to `readonly` (shallow one level only),
  to avoid accidental assignment to `props.foo` (usually a getter)
  while still allowing passing mutables in props.
  Add `Props<T>` helper for this transformation.
* Add @lxsmnsyc's `Ref<T>` type so it's easy to type `props.ref`:
  `Component<{ref: Ref<Element>}>`.
  <solidjs#778 (comment)>
  Fixes solidjs#778.

Comments from Otonashi