memoryMailBox uses multiple container/list.List objects to track
messages and packets, which use interface{} to accept objects of any
type. go1.18 added generics to the language, which means we could use a
typed list instead, allowing us to stop using forced type assertions
when reading objects from the list.
I'm not aware of any standard library implementation of a typed list yet,
so let's just add a TODO for now.
b1a3c46759