lib,src: support values > 4GB in heap statistics

We were transporting the heap statistics as uint32 values to JS land but
those wrap around for values > 4 GB.  Use 64 bits floats instead, those
should last us a while.

Fixes: https://github.com/nodejs/node/issues/10185
PR-URL: https://github.com/nodejs/node/pull/10186


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>