Provide hooks for message-listener callbacks

[ecpeterson]

A lot of simulation processing power is spent on PROCESSes which are busy-waiting for the receipt for some message. This could be alleviated by providing a callback to the COURIER which is triggered when an appropriate message arrives.

This comes with several difficulties: a PROCESS has both passive listeners (as installed in its message dispatch table) which are selectively triggered (via their guards) as well as active listeners installed via commands. The longevity of either of these classes is questionable. Additionally, callbacks will have strange timing properties: what hardware-realistic value would time have when a callback is triggered? (Recently, there's also the added complication of peruse-inbox?.)