fix(types): Datum.toComponent() should return ReactElement instead of Component

[Josh-Cena]

React.Component is something like function Comp() {...} or class Comp extends Component, while React.ReactElement is like <Comp />, the "return value" of a component. This has blocked me from doing something like React.Children.toArray(datum).

[Josh-Cena]

@staylor Could you take a look at this, please?

[OliverJAsh]

FWIW the same fix was made to react-helmet. https://github.com/DefinitelyTyped/DefinitelyTyped/commit/b0ff447be35d1e3ca9b4b41e66ec55059f6dc17e