Add default task pools and method for configuring them#2

Merged

lachlansneff merged 3 commits intolachlansneff:bevy-tasksfrom aclysma:bevy-tasks

Aug 28, 2020

+521-378

Commits on Aug 25, 2020

Remove generics from TaskPool newtypes and some of the task API
aclysma
committed
Pull TaskPool and related types out of bevy_tasks/lib.rs into a separate module
aclysma
committed

Commits on Aug 28, 2020

Assert percent of cores >= 0
aclysma
committed