buffer: implement Uint8Array backed Buffer

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.

PR-URL: https://github.com/nodejs/io.js/pull/1825


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>