FBVector 2.0 - now standard compliant

FBVector 2.0 - now standard compliant

Summary:
This is FBVector 2.0. It supports all of the original FBVector
optimizations and is standard compliant.
Accompanying this diff are two suites, StlVectorTest and Benchmark.
StlVectorTest runs an extensive battery of tests against a vector
implementation per the N3337 standard. In addition to checking normal
correctness, StlVectorTest checks the use of allocators, exception
safety, memory leaks, and type requirements.
Benchmark run a wide range of speed tests between std::vector, the
original fbvector, and the new fbvector.

Test Plan:
First test: run StlVectorTest.
Second test: run Benchmark.
Third test: compile and run some fbcode (e.g. multifeed/).

Reviewed By: andrei.alexandrescu@fb.com

FB internal diff: D566719