Use more compact keys, prefer Map to Object

[perliedman]

Hi, first off let me just thank you for an amazing project! I've had so much fun rendering voxel landscapes, a breeze to get started with this. Of course I more or less immediately started to push this to its limits, and encountered some problems when generating large maps.

I think I found some potential improvements:

• Most importantly, using ES6 Map instead of Object to store the voxels and their index appears to be much faster
• I also made some changes regarding how keys are generated, preferring integers (as much as they exist in JavaScript...) or at least shorter strings as keys to the maps
• Material information was duplicated in the Stage as well as in the VoxelIndex, so I replaced this with a reference from the Stage to the VoxelIndex, which saves memory

Using this, I have shaved off something like 40% of the heap size when I generate a 512x32x512 Vixel, and time to initialize the map is down from 8 seconds to 6.7, which is not as significant but at least better.

Hope you can find some use for these changes!

[wwwtyro]

@perliedman Thank you for these. Sorry it's taken me a year and a half to take a look. I'll see about integrating them.

[perliedman]

No worries, I'm more than familiar with the feeling 😅