also reduce the aggressiveness of std.atomic.stack
and std.atomic.queue fuzz testing. appveyor has 1 core
and 10,000 iterations is too much for 6 threads to
thrash over
* remove std.os.spawnThreadAllocator - windows does not support
an explicit stack, so using an allocator for a thread stack
space does not work.
* std.os.spawnThread - instead of accepting a stack argument, the
implementation will directly allocate using OS-specific APIs.
Andrew Kelley