Deprecate class shadowing

[adriaanm]

or should this wait until 2.14? (I think I had a ticket on this already but can't find it.)

[dwijnand]

What's meant by class shadowing?

[adriaanm]

class A { class C } ; class B extends A { class C } // A#C is shadowed in B

[adriaanm]

There's a ticket somewhere that shows how we get the two Cs confused. Unsoundness ensues.

[smarter]

See scala/scala3#4361

[smarter]

Duplicate of https://github.com/scala/scala-dev/issues/156