Generating array types like [4]int would fail even though the int type
is generatable. Allow generating values of array types when the inner
type is generatable.

Change-Id: I7d71b3c18edb3737e2fec1ddf5e36c9dc8401971
Reviewed-on: https://go-review.googlesource.com/3865Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>