convert timer device to "raw device", remove userspace wiring requirement

[warner]

As mentioned in #1346, when we next do significant work on the Timer device (perhaps because of #4282 or #4286), we should consider converting it to a "raw device", enhancing deviceTools.js to make it easier to manipulate Promises, and merge the companion vat (src/devices/timer.js and src/devices/timer-src.js). This would remove the unfortunate requirement that userspace bootstrap() glue the timer device to the timer vat.

One open question is whether userspace should then use vats.timer or devices.timer. The former is probably cleaner, but would probably require a lot of userspace code to be modified.

We'd want userspace to continue to be able to make E() calls to use the timer, which means enhancing raw devices to handle object references well. That might be a larger task than simply exposing device nodes that could work with promises.

[warner]

It'd be nice to do this, but it doesn't help userspace much, so I can't justify including it in MN-1.