Add chunking to ob_start

Summary: We were ignoring the `chunk_size` when provided to `ob_start`. This is an attempt
at an implementation of chunking built on our current output buffering infra.

Fixes #3820

Reviewed By: @paulbiss

Differential Revision: D1679049

Signature: t1:1679049:1416550346:fb3ce78f13b2e89deea926b1a581a263bf41a2fc
Allow null to password_hash()

Summary: PHP 5.6 allows `null` to be passed to `password_hash()`. Now we can pass it.

This is only a partial fix for #4294 since `password_hash()` also allows `int`s that can be
converted to `string` as well. Instead of making the parameter a `mixed`, we will wait for
`ParamCoerceMode` (PCM) to land, which should take care of that automatically for us.

Reviewed By: @paulbiss

Differential Revision: D1696229

Signature: t1:1696229:1416537939:b777eeda6f18173c356b950b48eb06cb0e31a7ed