Allowing the name to be passed to the ARGS_THIS macro will make it
easier to share code with the Uint8Array implementation.

With V8 4.4 removing the external array data API currently used by
Buffer, the new implementation uses the Uint8Array to back Buffer.

Buffers now have a maximum size of Smi::kMaxLength, as defined by V8.
Which is ~2 GB on 64 bit and ~1 GB on 32 bit.

The flag --use-old-buffer allows using the old Buffer implementation.
This flag will be removed once V8 4.4 has landed.

The two JS Buffer implementations have been split into two files for
simplicity.

Use getter to return expected .parent/.offset values for backwards
compatibility.

Passing a FreeCallback to Buffer::New() now uses externalized
ArrayBuffer's.

Use the new Maybe<> syntax for v8::Object::SetPrototype().

* Remove the deprecated API.
* Change Buffer::New() that did a copy of the data to Buffer::Copy()
* Change Buffer::Use() to Buffer::New()

New Buffer implementation allows greater than kMaxLength to be created.
So instead check if the passed value is a valid Smi.