Use (eval ...) to compile sim fns

[dgrnbrg]

Currently, sim fns use a map in a binding to do state argument lookup, and they call through many levels of fns. This could be made much faster by using eval to make a custom fn that does exactly what the expr needs at make-sim time.

[dgrnbrg]

One key usage of this is to make the mux2 and case branches lazily evaluated. When this is done, get-value for unions can throw exceptions during sims again (currently returns nil)

[dgrnbrg]

This will come later in a different form. Once the verilog generator is done, i'll use the same patterns to create a simulation code optimizer.