std::rt: Improve the error message when the thread-local ptr is null

Also fix some incorrect comments and variable names.
std::rt: Run the tests for Local in a bare thread

Otherwise interferes with the existing runtime
std::rt: 3MB stacks!

rustc needs *even more* megabytes when run without optimizations
std::rt: Don't allow schedulers to exit before handling all messages

Every time run_sched_once performs a 'scheduling action' it needs to guarantee
that it runs at least one more time, so enqueue another run_sched_once callback.
The primary reason it needs to do this is because not all async callbacks
are guaranteed to run, it's only guaranteed that *a* callback will run after
enqueing one - some may get dropped.

At the moment this means we wastefully create lots of callbacks to ensure that
there will *definitely* be a callback queued up to continue running the scheduler.
The logic really needs to be tightened up here.
std::rt: Remove the test for context()

This is no longer testable once newsched is turned on
Remove old tests and code for `select`

Not compatible with newsched
auto merge of #8282 : brson/rust/more-newsched-fixes, r=brson