This patch increases the performance of merge and filter. There are also some various other changes for minor performance increases or increased readability, and a collect function- a version of map without flattening, which would be more useful for users than the current map (not sure if it would be useful internally)

The merge change is to simply go two steps at a time instead of one. The filter saves the evaled functions. Note that two variable had to be given increased scope. If this is unacceptable, we can discuss ways to hide them.