remove performance directory
some improved spinlock
yield thief thread before work-stealing
fix scheduler::instance()
protect mutex' with spunlock only
Merge pull request #5 from nat-goodspeed/fiber-mt

Fiber mt
Merge pull request #6 from nat-goodspeed/fiber-mt

More thorough pass at introducing internal cross-referencing for Boost.Fiber doc.
fix round-robin for asio
changed interface of (un)bounded_queue
Fix doc about fiber::operator safe_bool(): NOT same as joinable().
Looking at the implementation, it seems that operator safe_bool() and its
companion, operator!(), interrogate whether the referenced fiber has or has
not terminated. joinable() does not.
Add more joinable() cross-references, and cross-reference joinable(), operator
safe_bool() and operator!().