Allow trialing comma in type list in HHVM

Summary: As per title. This give really confusing error messages, and is accepted by hh.

See https://www.facebook.com/groups/hackforhiphop/permalink/689199427795342/

Reviewed By: @elgenie

Differential Revision: D1393253
IteratorIterator->current() is supposed to return null after end of iterator

Summary: Fixes #2993
Closes #2996

Reviewed By: @ptarjan

Differential Revision: D1397183

Pulled By: @JoelMarcey