memoize: add test to document serialize/unserialize brokenness

Summary: ... the behavior is rather hilarious :D

Reviewed By: @​psaltzman

Differential Revision: D1627177
memoize: ensure that memoize caches aren't serialized

Summary: .. by reporposing ## AttrBuiltin ## on properties as a flag
 for properties internal to the runtime, thereby excluding them from
 serialization

Reviewed By: @​psaltzman

Differential Revision: D1627415
Fix OSS build - Remove extra OR operator

Summary: D1627415 introduced an extra `|` operator that the OSS compiler does not like:

```
/tmp/hhvm_oss_sync_and_push/hhvm/hphp/runtime/base/object-data.cpp: In member function ‘void HPHP::ObjectData::getProp(const HPHP::Class*, bool, const HPHP::PreClass::Prop*, HPHP::Array&, std::vector<bool>&) const’:
/tmp/hhvm_oss_sync_and_push/hhvm/hphp/runtime/base/object-data.cpp:1721:9: error: expected primary-expression before ‘|’ token
         | AttrBuiltin // runtime-internal attrs, such as the <<Memoize>> cache
```

Reviewed By: @ptarjan, @paulbiss

Differential Revision: D1628431
Adjust PackedArray allocation policy

Summary: Try growing a packed array in place before reallocating memory, by taking advantage of the request-local memory manager allocation policy.
Maximize the initial capacity in emitNewArray if the capacity is within a threshold.

Reviewed By: @jasone

Differential Revision: D1601707