Add SendDeferred trait and use it to fix #8214.
Fix nasty double-free bug where a newrt chan could get killed after rescheduling but before suppressing_finalize.
Fix embarrassing bug where 'unkillable' would unwind improperly when it receives a kill signal.
(cleanup) Use more do...finally in extra::sync.
Don't fail from kill signals if already unwinding.
Add an assert_may_sleep() check on every context switch.
auto merge of #8234 : bblum/rust/assorted-fixes, r=brson

This fixes 4 bugs that prevented the extra::arc and extra::sync tests from passing on the new runtime.

* In ```Add SendDeferred trait``` I add a non-rescheduling ```send_deferred``` method to our various channel types. The ```extra::sync``` concurrency primitives need this guarantee so they can send while inside of an exclusive. (This fixes deterministic deadlocks seen with ```RUST_THREADS=1```.)
* In "Fix nasty double-free bug" I make sure that a ```ChanOne``` suppresses_finalize *before* rescheduling away to the receiver, so in case it gets a kill signal upon coming back, the destructor is inhibited as desired. (This is pretty uncommon on multiple CPUs but showed up always with ```RUST_THREADS=1```.)
* In ```Fix embarrassing bug where 'unkillable' would unwind improperly``` I make sure the task's unkillable counter stays consistent when a kill signal is received right at the start of an unkillable section. (This is a very uncommon race and can only occur with multiple CPUs.)
* In ```Don't fail from kill signals if already unwinding``` I do pretty much what it says on the tin. Surprising that it took the whole suite of sync/arc tests to expose this.

The other two commits are cleanup.

r @brson