Create comp_execute

comp_execute() would be a function which accepts a single, constant iclosure, which is then fully resolved at compile time. The return value (which should be non-void) will replace the call to comp_execute in the parse tree.

One undiscussed issue is how values that are non-constant being returned should work. Since the value is replaced into the parse tree, what happens if something like an array or other mutable object is returned?