Sealed types -- updated proposal

[Brian Goetz]

> Given that the proposal introduce the notion of sealed types, "sealed" is a better keyword.

Note that `sealed` already has a meaning in the context of packages (see Package.isSealed()), though it is minor.

Is there a different hyphenation of final other than semi-final that maintains the connection to finality, but doesn’t weird people out?  (One thing I dislike about sealed / non-sealed is that now we have _contextual_ keywords with hyphens, which wasn’t the discipline we were aiming for in the hyphenation proposal.)