Revamped concurrency API

[Bartosz Milewski]

Nick B Wrote:
> 
> > Could you give us _any_ kind of test case (even if it's enormous)?
> 
> Bartosz - are you able to provide a test case as requested by Don ?
> Then it might be possible, to get this bug fixed.
> 
> Nick B.

I can send you the files I have checked out. 

The problem was in core.thread. I tried to implement a struct Tid (thread ID) with reference-counting semantics and deterministic destruction. It passed all the tests, but when it was used in one particular place in druntime it produced incorrect assembly. Even the slightest change made the bug disappear, so I wasn't able to reproduce it under controlled conditions.

Unfortunately, I have undone some of my changes trying to bypass the bug, so at the moment I don't even have the buggy version, but it can be reconstructed. We can discuss it off-line, if you want. Use my email address with -nospam removed.