src: move ParseArrayIndex() to src/node_buffer.cc

It's not used anywhere else so move it out of src/node_internals.h.

PR-URL: https://github.com/nodejs/node/pull/7497


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>