introduce a no collections mode

Summary: Most of the transformations of h2tp that result in code that is
very different from the input has to do with collections. This introduces a mode to
disable all collection related transformations
Fixes https://github.com/facebook/hhvm/issues/4225

Reviewed By: @jwatzman

Differential Revision: D1677973

Signature: t1:1677973:1416015234:220f58d20b5cbe854853174906a5923555766c0a