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

[Martin Buchholz]

The needToWorkAround6260652 changes ought to be in a separate changeset.

The biggest question is whether Collection.toArray(generator) pulls its
weight, especially in view of
https://shipilev.net/blog/2016/arrays-wisdom-ancients.

I rarely want to dump elements into a typed array.  Dumping into Object[]
with toArray() is just fine for me (but I'm a biased core library
developer).