Vat trace stepper

This PR adds two new meta-commands: ,vat-up and ,vat-down that move up/down the events in the vat trace currently being debugged. ,vat-peek-past is renamed to ,vat-peek and uses the current event as the context in which it is peeking. To make this happen, we slap a little more mud onto the ball of mud that is the exception generated within a vat turn. By including the vat event in the exception we can easily get the vat trace we need to debug.

Fixes #75 (closed)