layout: ARC the screen size, so that it can be mutated while layout
worker threads are holding onto it
layout: Implement parallel reflow for the bubble-widths and
assign-heights phases.

This uses the new work-stealing deque. By default, 3/4 of a thread per
logical CPU is used. This can be tuned with the `-y` flag.

I measured a 43% reflow speedup on `perf-rainbow.html` and a 247% reflow
speedup on `http://en.wikipedia.org/wiki/South_China_Sea` on a 4-core
HyperThreaded Core i7. However, numbers were fairly volatile, especially
for the latter.