[WIP] Abbandon numeric pids (implementation)

This is a quick WIP sketch of how I think that we can omit numeric PIDs.
I only made the hello_world example and the test_shell test running, and only on native.

I found that some structural things should be changed before work on #1004 can be done.
Especially I'd like to highlight my pending PRs #945 and #927 which would make further changes easier.