* Add React.isValidElementType()
Per the conversation on #12453, there are a number of third-party
libraries (particularly those that generate higher-order components)
that are performing suboptimal validation of element types.
This commit exposes a function that can perform the desired check
without depending upon React internals.
* Move isValidElementType to shared/
Is this necessary? I'd like to use the package in enzyme to avoid having to recopy/paste the symbols for better debugging names, but at hard dep in enzyme proper on a version of react isn't gonna work. This seems safe since nothing explicitly depends on React in here?
* Support ForwardRef type of work in TestRenderer and ShallowRenderer.
* Release script now updates inter-package dependencies too (e.g. react-test-renderer depends on react-is).
