[BREAKING] Add .root() and make shallow() and mount() semantics consistent

[lelandrichardson]

to: @ljharb

This PR updates both mount() and shallow() to have similar semantics with respect to what the wrapper that is returned by them.

The idea is that mount(<Foo />) and shallow(<Foo />) both return a wrapper that's around the node that Foo rendered, and not <Foo /> itself. This means that calling .type() and .instance() on the resulting wrapper will not return Foo and an instance of Foo, but the type and instance of the root node returned by it's render function.

Additionally, calling .root() on the wrapper will return a new wrapper that is around the node of <Foo />. Calling .type() on this wrapper will return Foo and .instance() will return the Foo component instance.

This is a huge breaking change, and i'm not convinced that this is the right API. But putting up this PR for comments.

[ljharb]

Let's chat more about this API and alternatives; but tbh I think this looks great.