DM-1128

New PR for code review of DM-1128, span-based grow operations for Footprint
"growFootprintRLE" isn't actually a valid name according to our standards - we require acronyms to be camelcased as well, so it'd have to be "growFootprintRle".  That's horrible, admittedly, and an example of why I'm not fond of that particular rule.  Given that this is now the only implementation for growing Footprints, maybe just rename it to growFootprintImpl?