Structuring and documenting top-level code

[matejpavlovic]

The code at the top level (mirbft.go mostly) needs to be documented and structured to an easy-to-understand form, probably involving splitting it in multiple files or even packages.

[matejpavlovic]

The branch top-level-refactor implements those changes now. mirbft.go does not exist any more and most of the code is better structured and documented.