Make Vector use a packed array for its buffer

Summary: This diff changes Vector to use a PackedArray for its buffer instead of
using its own custom buffers.

This diff is based on top of D1337469.

Reviewed By: @elgenie

Differential Revision: D1335725
Fix printir for AFrozen

Summary: With TRACE=printir, code emitted into AFrozen was not getting
printed. Fix this. Also CodeCache::frozen() should always be
m_frozen.

Reviewed By: @swtaarrs

Differential Revision: D1361980