Add <<__NativeData("Type")>> registry for over-allocating ObjectData

Add <<__NativeData("Type")>> registry for over-allocating ObjectData

Allows storing arbitrary data in an ObjectData structure
from extension code.

Allocates extra space (as requested by Native::registerNativeDataInfo())
in the ObjectData sturcture (prior to the formal struct) and calls
Init, Clone, and Destroy functions (as registered).

Standard class implementation implement these via their empty constructor,
copy constructor, and destructor respectively.

For example:

ext_foo.php
  <?hh
  <<__NativeData("MyClass")>>
  class MyClass {
    /* blah blah blah */
  }

ext_foo.cpp
  struct MyClass : Native::NativeData {
   public:
    // Called by `$x = new MyClass;` or `$x = clone $MyClassInstance;`
    // to initialize the C++ object
    MyClass() {}
    // Called by `$x = clone $MyClassInstance`
    // to copy from an existing C++ object
    MyClass& operator=(const MyClass& src) {
      if (src.m_foo) {
        m_foo = newFoo(src.m_foo);
      }
      return *this;
    }
    // Called when object goes out of scope or request end
    ~MyClass() {
      if (m_foo) {
        delete m_foo;
      }
    }

    // etc...

   private:
    Foo *m_foo = nullptr;
  }

Further down in the Extension structure:
  void moduleInit() override {
    Native::registerNativeDataInfo<MyClass>(s_MyClass.get());
  }

Reviewed By: @jdelong

Differential Revision: D1171055