[pattern-switch] Opting into totality

[Brian Goetz]

> So I appreciate the brainstorming, but personally don't really see a problem that needs solving here. (Other than the syntax bikeshed.)

Agreed, it was worth poking around the corners with a flashlight, but 
the exploration brought me back to this point too.  (If we even need it 
at all.)

Some of the motivation for this exercise has been that some people feel 
uncomfortable at inferred totality.  But, there's a good chance that 
this discomfort is temporary -- this happens every time any sort of 
implicitness is added to the language.   (People freaked out about var 
at first too.)