layout: Port parallel layout over to a generic "work queue"
infrastructure.

The work queue accepts abstract generic "work units", which in this case
are layout operations. The same speedups have been observed.
auto merge of #1551 : pcwalton/servo/generic-parallel-layout, r=larsbergstrom

infrastructure.

The work queue accepts abstract generic "work units", which in this case
are layout operations. The same speedups have been observed.

r? @larsbergstrom