Support nested Switches

Hi, I've read the docs to see if nested Switches are supported (a longstanding wish), and I think they are not, so I wanted to make a feature proposal.

The idea is that a switch picks the first matching route or switch. A switch matches if one of its routes (or switches) matches. It's basically the Composite pattern.

The use-case is this one:

<Switch>
    <Route path="blue/one"/>
    <Route path="red/one"/>
    <Route path="bread/one"/>
    <Route path="apple/one"/>
</Switch>

If I want to create separate route groups for "color" or "food" then it would be great if I can refactor this to

// Switch that renders either ColorsSwitch or FoodsSwitch.
// This approach can be used to clean up switches with many routes.
// It can also be used to reuse switches in different locations.
<Switch>
    <ColorsSwitch/>
    <FoodsSwitch/>
</Switch>

const ColorsSwitch = () => {
    return (
        <Switch>
            <Route path="blue/one"/>
            <Route path="red/one"/>
        </Switch>
    )
}

const FoodsSwitch = () => {
    return (
        <Switch>
            <Route path="bread/one"/>
            <Route path="apple/one"/>
        </Switch>
    )
}

The issue names only the Switch and Route components, not source files or tests. Start by locating their implementation and existing routing tests; done means nested switches can be composed and select the first matching route or switch, with tests covering the grouped example.