Macro define_implementation 

macro_rules! define_implementation {
    { $mapname:ident } => { ... };
}

Helper: Define a wrapper for a single SlotMap type.

This works for SlotMap, and DenseSlotMap.

(The alternative to using a macro here would be to define a new trait implemented by all of the SlotMaps, and then to define our own SlotMap as a wrapper around an instance of that trait.)