vat: generate

2005-09-21: Implemented the "Generate" module which generates random function trees for an initial population.

Some random functions, and their corresponding trees:
(three manually chosen from sixty)

render for seed=02 tree for seed=02
render for seed=26 tree for seed=26

render for seed=43 tree for seed=43

My previous attempt at vat evaluated functions in the range [0,1] whereas now I'm using [-1,1]. Between that and the current choice of functions, it makes sense that the output tends to look symmetric.