Add new quick hull benchmark

Summary: Added a new benchmark named QuickHull that computes the convex hull of
1000 points using the quick hull algorithm (see:
http://stackoverflow.com/a/18418182). The benchmark leans heavily on
struct-like arrays for representing points and line equations. It also
makes frequent use of some array_* library intrinsics (e.g. array_reduce).

Reviewed By: @jdelong

Differential Revision: D1573015
Fix LowStringPtr pretty printer

Summary: My LowStringPtrs have always printed out as "LowStringPtr '(type str)'"
and I just figured out why.

Reviewed By: @mxw

Differential Revision: D1578616