refactor sidechain writing function

[mixmix]

I refactored this code on a call with @nanomonkey - we're using it to try and write a spec, and found it very hard to read in it's native state. This PR is opened so that

• we have a clearer reference for spec writing
• we have an example of what more accessible code looks like (IMO)
  • comments
  • grouping of code into more coherent chunks
    • collecting related code
    • putting white space in to aid reading

🔥 TODO before merge:

• write tests over this function (check they pass before + after)
• resolve duplication across simplechat + simplepub folders 😭