Optimise struct fields for better memory alignment

[mochi-co]

Following #17 we found that various struct fields were not 8bit aligned, which resulted in panics on 32bit builds. Fixing this has highlighted that there are other structs within the code which can be optimised to have better memory alignment.

These should be refactored so that each struct uses the optimal amount of memory for better caching, and that all fields should be 8bit aligned for improved 32bit compatibility.