Performance profiling and optimisation

[Libbum]

There are most likely performance enhancements that can be made which I've not investigated all that thoroughly. Now that the library is effectively 'complete', we can try to shave off some cycles.

[Libbum]

1e04059 is a start.

Not sure if rayon is going to help at all much in the current format - my own tests and #11 haven't yielded too much success. Probably need to speak to an expert about the memory management.