RFR(s): 8060192: Add default method Collection.toArray(generator)

[Stuart Marks]

Revised webrev based on comments from Martin Buchholz and Paul Sandoz:

     http://cr.openjdk.java.net/~smarks/reviews/8060192/webrev.1/

I've done a bit of rewriting of the @apiNote sections to clarify the intended 
use of the various toArray() methods.

s'marks