refactor: make string formatting more readable

To make the final output code easier to see:
* Get rid of the unnecessary line-noise of `.unwrap()`ing calls to
  `write!()` by moving the `.unwrap()` into a macro.
* Join consecutive `write!()` calls using a single multiline format
  string.
* Replace `.push()` and `.push_str(format!())` with `write!()`.
* If after doing all of the above, there is only a single `write!()`
  call in the function, just construct the string directly with
  `format!()`.

Author: Kmeakin