small_vector exception safety, part 2

small_vector exception safety, part 2

Summary:
small_vector is now object-exception safe for all container
functions, except for input-iterators.

That's a bold claim; probably incorrect. At the very least, it passes
the same test suite as std::vector and fbvector.

Aside: Clearly, no one uses erase(q1, q2) in the wild.

Facebook: Nothing special.

Test Plan:
fbconfig -r folly && fbmake runtests
fbconfig -r experimental/njormrod/stltest && fbmake runtests

Reviewed By: andrei.alexandrescu@fb.com

FB internal diff: D1319787