Combines a refactor of the runner module and a bit of the session
module for clarity and maintainability, and a fix for #21.

Fixes #21

… and remove the work-around for
https://github.com/python/mypy/issues/14242

Closes #22

kilter.protocol 0.6 makes a change that necessitates an assertion that
messages with bytes data exclusively use memoryviews, at least until
`freeze()` is called on them.

There should be a typing solution to this problem but it may have it's
own issues.

For real this time, hopefully. At least the tests pass now.

`ChangeHeader` index values are for differeniating between headers with
the same name. They are not the absolute index position of the header
within the full header list. They are also 1-based.

https://pythonhosted.org/pymilter/milter_api/smfi_chgheader.html