const'ing sorted_vector_map::count()

Test Plan: Use it

Reviewed By: tudorb@fb.com

FB internal diff: D865211
Multi-producer multi-consumer queue with optional blocking

Summary:
MPMCQueue<T> is a high-performance bounded concurrent queue that
supports multiple producers, multiple consumers, and optional blocking.
The queue has a fixed capacity, for which all memory will be allocated
up front.  The bulk of the work of enqueuing and dequeuing can be
performed in parallel.

To make an MPMCQueue<T>, T must satisfy either of two conditions:
- it has been tagged FOLLY_ASSUME_FBVECTOR_COMPATIBLE; or
- both the constructor used during enqueue and the move operator are
marked noexcept.

This diff extracts the generic component from tao/queues/ConcurrentQueue
and renames identifiers to match those of existing folly queues.
It also includes an extraction of Futex, which wraps the futex syscall,
and DeterministicScheduler, which allows for deterministic exploration
of thread interleavings for components built from std::atomic and Futex.

Test Plan: new unit tests

Reviewed By: tudorb@fb.com

FB internal diff: D866566
Range<T>::rfind()

Test Plan: Unit tests

Reviewed By: andrei.alexandrescu@fb.com

FB internal diff: D865951
toDynamic(T)

Summary: It only makes sense to be able to go the other direction, too.

Test Plan: Unit tests

Reviewed By: delong.j@fb.com

FB internal diff: D785282
strerror_r is XSI compliant on Apple/FreeBSD

Summary:
- Noticed this due to an -fpermissive compiler warning while compiling
HHVM for OSX (complaint of trying to cast int to char*)

Test Plan:
- Compiled
- Confirmed the build warning is fixed by this on the Mac OS X build

Reviewed By: tudorb@fb.com

FB internal diff: D865169