Turn off MemeLim store sinking for now (it isn't right)

Summary: This is currently willing to move stores to places where the value
that would've been in the memory location is different.  We need to limit store
motion to blocks dominated by the old store, not just dominated by the value
that was stored, or do a more general version of this.

Reviewed By: @markw65

Differential Revision: D1654195
Fribidi extension for HHVM

Summary: Kind of a noob to this stuff but this is a straight port of pecl-fribidi.  I have some concerns over how the strings are being managed...  for a PHP extension you can specify whether or not the string being returned should be copied.  Not sure how to do that for HHVM.

Reviewed By: @ptarjan, @behnam

Differential Revision: D1632647
Optimization for non-typehinted functions

Summary: For functions with args and nullable return or no typehint, we need to do a slightly slower check of the cache to guarantee we're correct. However, async functions never return null since they always return awaitable. This lets us use the faster check for memoized async functions with args and no return type hint. There aren't very many of those in prod right now but there are a bunch in the codemod.

Reviewed By: @jano

Differential Revision: D1652958

Signature: t1:1652958:1414790937:58ac19de0cc39ee0887b9d54de0bad167a45a924