Update crossbeam-deque

[ghost]

Update crossbeam-deque to 0.5.0.

This version has better-optimized support for deques that process tasks in FIFO order (previously we'd call Deque::steal() to pop tasks from the opposite end of Deque::push(), which is sort of a hack).

Moreover, some internal fields of the deque are now directly inlined into the worker side in order to avoid false sharing. (cc #422)

Benchmark results:

 name                    before ns/iter  after ns/iter  diff ns/iter  diff %  speedup
 threadpool::spawn_many  4,633,930       4,216,334          -417,596  -9.01%   x 1.10
 threadpool::yield_many  11,931,281      12,263,546          332,265   2.78%   x 0.97

Closes #329.