Decide what compiled operators look like

[mortendahl]

Compiling an operator currently results in a Box<dyn Vec<C::Value>> -> C::Value, which unlike the older code base doesn't allow operations to do much compile-time checking (eg arity). Moreover, always creating a Box has some performance overhead that rustc seems unable to remove.

This issue is about deciding whether we want to use more typed objects to represent compiled operators, eg an enum.