Signal-trap table

[graydon]

OS signals currently don't get trapped properly, nor do we have a timer thread that interrupts a runaway task. Add such, using a flag to indicate a pending signal. Put a check at preemption points (fn entries and loop edges).

[graydon]

Added preemption points and a timer. No other-kinds of signals yet.

[graydon]

Obsolete. We'll use uv to pick up OS signals (hopefully!) and and explicit communication primitives for this.