Don't reseed PHP's rand & mt_rand on instantiation.

It's just a waste of time to do it by default.  Call it explicitly if needed.
Fix SecureRandom::get_integer.

- Allow arbitrary ranges of signed integers.
- Handle both 32- and 64-bit (and possibly other sizes too).
- Don't overcomplicate things trying to be clever about prefetching
  more random bits in advance.  By the way, THERE WAS A BUG.
- Handle long "unwrappable" ranges separately to simplify (otherwise
  signedness makes things hard).