src: move internal functions out of node_buffer.h

The circular dependency problem that put them there in the first place
is no longer an issue.  Move them out of the public node_buffer.h header
and into the private node_internals.h header.

Fixes: https://github.com/nodejs/node/issues/2308
PR-URL: https://github.com/nodejs/node/pull/2352


Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>