[switch] Further unification on switch

[Brian Goetz]

> Here's a fun statistic from Google's codebase. We analyzed every 
> hand-written switch statement in our depot. Only /2.4%/ of them used 
> fall-through. The number in the real world might be somewhat higher 
> (?), but we know it to be quite small. Yet, the things about switch 
> that have been weird, confusing, or dangerous all stem from that 
> fall-through model that such a tiny fraction need -- how sad!

We did a similar (though less methodical) survey of switches in 
java.base, and found a higher number, but that's to be expected because 
(a) its an old code base that is closer to the C influences and (b) it 
contains a lot of low-level code for parsing packets and such.  But my 
general conclusion is the same.