`rust` branch tracking issue

[wincent]

Just some known things I want to get to:

• [x] Implement histogram diff.
• [ ] Implement optimizations for Myers diff (it's still used as a fallback for the histogram diff, so should be fast).
• [x] Implement fast hashing function and precompute hashes before calling internal diffing functions.
• [ ] Try to reduce repetitive casts usize ↔︎ isize that are all over the place.
• [x] Improve snapshot diff output (for example, not currently printing hunk headers or limiting context lines).
• [ ] Finish Lua parser.
• [ ] Make pretty lexer/parser errors (ie. transform kind: UnexpectedToken, position: 14 into something human-useful).
• [ ] Make Vimscript lexer + parser.
• [ ] Make output pretty-printer.
• [ ] Implement benchmarks.
• [ ] Clean up tire-fire that is the RingBuffer implementation.