Potential optimisations & refinements

[tonyarnold]

@wokalski identified potential optimisations and refinements that could be made to this library - I'd be grateful for contributions if anyone feels like tackling any of these, but mainly I'm just making sure they're documented for future me:

• [ ] https://neil.fraser.name/writing/diff/ (from https://github.com/wokalski/Diff.swift/issues/5)
• [x] The current implementation is the simplest version of Myers' algorithm. Linear space refinement as described in the paper would improve our implementation. (from https://github.com/wokalski/Diff.swift/issues/6)