Reduce cost of control parameter initialization

[chrispcampbell]

In the fix for #301, in order to initialize the control parameters (in the accessors used by the runtime packages), we are now calling initConstants, initLevels, and evalAux, which means we are executing many unnecessary statements.

We should consider changing the code gen to initialize the control parameters separately. We should also consider compile-time validation of the control parameters to see if they can be resolved to a constant value.

There is some overlap here with the API considerations being made in #302, so it would be good to ponder these two issues together.