Store turtles’ links for fast refreshLinks bailout

These should also be used to optimize connectedLinks as well, but tests fail when I do that so it needs more work.
Make collectUpdates return only a single update for all agent updates.

This is a huge speed improvement. Note that world does not use the new mechanism yet, but this doesn’t affect behavior.
Merge pull request #57 from NetLogo/optimize-climate

Optimize climate