ocamlbuild build for Hack open source

Summary: This replaces (most of) the open source build for Hack by a much simpler
ocamlbuild-based one.

Main points of interest:
* Build itself is straightforward and looks a lot like the Flow one
* I did NOT do the JS build - just fixed up an existing breakage. Not really sure how to make that one more maintainable
* Updated the test runner so we don't have to list test directories in two places

Reviewed By: @jwatzman

Differential Revision: D1654465

Signature: t1:1654465:1415402598:72396c4eac1cb02aa652b72ae9ff62a701571d22