Depending on bittedness of the runner, uint16 * uint16 can overflow an int.
There's other computations that can overflow a uint32 as well, so I just made
everything uint64 to avoid thinking about it too much.
Change-Id: Ia3c976987f39f78285c865a2d7688600d73c2514
Reviewed-on: https://boringssl-review.googlesource.com/8193
Reviewed-by: Adam Langley <agl@google.com>