Support to nest MemoryRouter with other routers

[VaZark]

On the website that I'm currently building, we use a form wizard within Modals. However, since I have a BrowserRouter globally, I am unable to use MemoryRouter within the app. Thus I end up needing to handle the routing and rendering for navigating by hand.

If MemoryRouter can be nested within other routers, this micro-navigation can be completely left up to react-router, and form wizards would be a breeze to build for everybody.

Since MemoryRouter handles history via arrays in-memory, this shouldn't impact application functionality either and can be updated without a breaking change.

Are there any specific reasons that the library has chosen not to allow it?

[brophdawg11]

Superseded by #9601