v8,src: expose statistics about heap spaces

Provide means to inspect information about the separate heap spaces
via a callable API. This is helpful to analyze memory issues.

Fixes: https://github.com/nodejs/node/issues/2079
PR-URL: https://github.com/nodejs/node/pull/4463


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>