A major refactoring that changes the way the runtime uses TLS. In the
old design the TLS held the scheduler struct, and the scheduler struct
held the active task. This posed all sorts of weird problems due to
how we wanted to use the contents of TLS. The cleaner approach is to
leave the active task in TLS and have the task hold the scheduler. To
make this work out the scheduler has to run inside a regular task, and
then once that is the case the context switching code is massively
simplified, as instead of three possible paths there is only one. The
logical flow is also easier to follow, as the scheduler struct acts
somewhat like a "token" indicating what is active.

These changes also necessitated changing a large number of runtime
tests, and rewriting most of the runtime testing helpers.

Polish level is "low", as I will very soon start on more scheduler
changes that will require wiping the polish off. That being said there
should be sufficient comments around anything complex to make this
entirely respectable as a standalone commit.
Have linked failure tests run on the new scheduler instead of requiring RUST_NEWRT to test.
Fixed a race where a scheduler configured to only run tasks pinned to it would "bounch" a regular task in and out of the work queue without allowing a different scheduler to run it.
minor tweaks - unboxed the coroutine so that it is no longer a ~ pointer inside the task struct, and also added an assert to verify that send is never called inside scheduler context as it is undefined (BROKEN) if that happens
fixed incorrect handling of returned scheduler option and restructed scheduler functions slightly
auto merge of #8116 : toddaaro/rust/tls-tk-pr-pre, r=brson

Merged with task killing code this time around.