Make encoding faster

[patrickmcnamara]

This library is very slow at encoding images. This should be changed.

I imagine the problem has something to do with the fact that we are doing an O(n) nearest neighbour search for every single pixel (lol). There has to be a better way.

I encoded a 205 MiB 30000x23756 RGB JPEG earlier and it took two hours and fifty minutes on my Ryzen 5 1600. My goal is to have that be under a minute.

[patrickmcnamara]

Current ideas on this:

• Use a minimal perfect hash function.
• Use a very large array and index it using RGB values.
• Use multiple CPUs.