Move ReactClass, ReactElement and ReactPropTypes into "classic"

This moves ReactClass, ReactElement and ReactPropTypes into a legacy folder
but since it's not quite legacy yet, I call it "classic".

These are "classic" because they are decoupled and can be replaced by
ES6 classes, JSX and Flow respectively.

This also extracts unit tests from ReactCompositeComponent, which was
terribly overloaded, into the new corresponding test suites.

There is one weird case for ReactContextValidator. This actually happens in
core, and technically belongs to ReactCompositeComponent. I'm not sure
we will be able to statically validate contexts so this might be a case
for dynamic checks even in the future. Leaving the unit tests in classic
until we can figure out what to do with them.

This is only moving tests around. No new tests nor changed behavior.