Write generic get/set messages

[ecpeterson]

In application code, we have a family of generic get/set messages, as well as a -> macro which performs repeated de/referencing. I would like aether to own these message definitions / handlers and for them to be as flexible as the -> version.

For instance, a get-> message might have as payload a list of lists of slot names. A process receiving such a message examines the first list of slot names, '(slot1 … slotN), and unwraps the value (slot-value (… (slot-value self 'slot1)…) 'slotN). If there are more slot lists, it expects this value to be an address, and it sends a new get-> message to that address with the remaining slot lists. If there are no more slot lists, it returns this value to the return address.

Whether a process services get/set messages is up to the author of that subclass's particular dispatch table.