Revert "Revert "Split & remove extension_parallel (#8983)"" (#9190)

This reverts commit 05a160ec67ceab3012a9e3bce3443445a6069bec.

Revert "Revert "Make serial parallel_for "polyfill" iterate backwards in
debug builds (#9044)""

This reverts commit 815eaff8e604d5a99d971f1241321616e09c2884.

Revert "Revert "Unbreak optimized kernels buck build (and check it in
unittest-buck) (#9159)""

This reverts commit 10bb615a17cc77dee9e448a78a992998076315d7.