When generating a random int8, uint8, int16, uint16, int32, uint32,
quick.Value chooses among all possible values.

But when generating a random int64 or uint64, it only chooses
values in the range [-2⁶², 2⁶²) (even for uint64).
It should, like for all the other integers, use the full range.

If it had, this would have caught golang#19807 earlier.
Instead it let us discover the presence of golang#19809.

While we are here, also make the default source of
randomness not completely deterministic.

Fixes golang#19808.

Change-Id: I070f852531c92b3670bd76523326c9132bfc9416
Reviewed-on: https://go-review.googlesource.com/39152
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>