Outstanding language changes

[odersky]

Here is a list of language changes that we committed to or are at least strongly considering. It would be good to get these sorted out quickly before we keep the feature envelope ready.

• Revert @volatile lazy vals. It changes runtime behavior, which we want to avoid. Make lazy vals thread-safe by default #6579 @smarter
• Drop nonlocal returns: Deprecate nonlocal returns #6623. @OlivierBlanvillain
• Allow enums to extend java.lang.Enum Java-compatible enums #6576 @anatoliykmetyuk
• Generalization of structural types -- deferred
• Implement @alpha Implement @alpha annotation #6597 @odersky
• Import implied Implement for clauses for implied imports #6594 @odersky