Optimization: a slot for the next task to run

[ghost]

This is an optimization from Go's scheduler, which has recently also been adoped by Tokio.

The idea is that each worker thread, besides holding a local task queue, also holds a slot of type Cell<Option<Runnable>> that contains the next task to run. When scheduling a new runnable task, we store it into the slot. If the slot already contained a task, we then push that task into the local task queue. This way we don't have to touch queues in many cases at all. Since queues are more expensive that task slots, this can drastically improve performance.

In Go's scheduler, runqput stores a task into this slot to schedule it, while runqget takes a task from the slot. The linked Tokio blog post illustrates this nicely with diagrams: take a look at the "Run next" slot that is positioned in front of "Run queue".

This PR also deleted some unsafe code by replacing UnsafeCell with OnceCell.

Benchmarks:

 name           before ns/iter  after ns/iter  diff ns/iter   diff %  speedup
 contention     983,233         270,738            -712,495  -72.46%   x 3.63
 create         4               4                         0    0.00%   x 1.00
 no_contention  271,792         215,953             -55,839  -20.54%   x 1.26

 name           before ns/iter  after ns/iter  diff ns/iter   diff %  speedup
 chained_spawn  119,929         121,418               1,489    1.24%   x 0.99
 ping_pong      256,357         259,504               3,147    1.23%   x 0.99
 spawn_many     2,728,784       2,842,761           113,977    4.18%   x 0.96
 yield_many     3,530,096       3,141,737          -388,359  -11.00%   x 1.12