Thread tree

A tree-structured thread pool.

Stack jobs and job execution based on rayon-core by Niko Matsakis and Josh Stone.

Experimental simple thread pool used for spawning stack-bound scoped jobs with no work stealing.

This is good for:

• You want to split work recursively in jobs that use approximately the same time.

This is not good for:

• You need work stealing
• When you have jobs of uneven size

Wild ideas and notes

Possibly allow reserving a subbranch of the tree.