BaseObject is a class that just handles the Persistent handle attached
to the class instance.

This also removed WeakObject. Reordering the inheritance chain helps
prevent unneeded calls on instances that don't call MakeCallback.

Making WeakObject inherit from AsyncWrap allows us to peak into almost
all the MakeCallback calls in Node internals.

Remove the NODE_{WRAP,UNWRAP} macros and instead use template functions.

Turns out that we don't use node_object_wrap.h any more in core,
and, with v8 3.21, it's breaking our Windows build. Removing refs
to it everywhere (and adding node.h in one case where it was the
only way node.h was being included), we have restored the Windows
build.

Drop the ObjectWrap dependency in favor of an internal WeakObject class.

Let's us stop worrying about API and ABI compatibility when making
changes to the way node.js deals with weakly persistent handles
internally.

Fix pointer unwrapping when T is a class with more than one base class.

Before this commit, the wrapped void* pointer was cast directly to T*
without going through ObjectWrap* first, possibly leading to a class
instance pointer that points to the wrong vtable.

This change required some cleanup in various files; some classes
used private rather than public inheritance, others didn't derive
from ObjectWrap at all...

Fixes #6188.

This commit makes it possible to use multiple V8 execution contexts
within a single event loop.  Put another way, handle and request wrap
objects now "remember" the context they belong to and switch back to
that context when the time comes to call into JS land.

This could have been done in a quick and hacky way by calling
v8::Object::GetCreationContext() on the wrap object right before
making a callback but that leaves a fairly wide margin for bugs.

Instead, we make the context explicit through a new Environment class
that encapsulates everything (or almost everything) that belongs to
the context.  Variables that used to be a static or a global are now
members of the aforementioned class.  An additional benefit is that
this approach should make it relatively straightforward to add full
isolate support in due course.

There is no JavaScript API yet but that will be added in the near
future.

This work was graciously sponsored by GitHub, Inc.

This is a big commit that touches just about every file in the src/
directory. The V8 API has changed in significant ways. The most
important changes are:

* Binding functions take a const v8::FunctionCallbackInfo<T>& argument
  rather than a const v8::Arguments& argument.

* Binding functions return void rather than v8::Handle<v8::Value>. The
  return value is returned with the args.GetReturnValue().Set() family
  of functions.

* v8::Persistent<T> no longer derives from v8::Handle<T> and no longer
  allows you to directly dereference the object that the persistent
  handle points to. This means that the common pattern of caching
  oft-used JS values in a persistent handle no longer quite works,
  you first need to reconstruct a v8::Local<T> from the persistent
  handle with the Local<T>::New(isolate, persistent) factory method.

A handful of (internal) convenience classes and functions have been
added to make dealing with the new API a little easier.

The most visible one is node::Cached<T>, which wraps a v8::Persistent<T>
with some template sugar. It can hold arbitrary types but so far it's
exclusively used for v8::Strings (which was by far the most commonly
cached handle type.)