buffer: expose internals on binding

Remove internal object and expose functions directly on binding.  This
makes possible to simply use internal functions in other builtin
modules.

PR-URL: https://github.com/iojs/io.js/pull/770


Reviewed-by: Trevor Norris <trev.norris@gmail.com>
Reviewed-by: Ben Noordhuis <info@bnoordhuis.nl>