New flagged storage for parallel joins#737

Draft

Imberflur wants to merge 3 commits intoamethyst:masterfrom

Imberflur:new-flagged-storage

Commits on Oct 28, 2021

Document stronger safety requirements for implementing and using Join::get that should eliminate unsoundness if followed, add extra safety requirement to UnprotectedStorage::get implementation, com…
Imberflur
committed
Add new flagged storage that allows soundly defering tracked mutable access of a component in a non streaming join as well as using par_join with tracked mutable access
Imberflur
committed

Commits on Oct 29, 2021

Improve the docs with respect to the new flagged storage variant and the current removal of restricted storage (and comment out an example that was using the restricted storage)
Imberflur
committed