Refactor linebreaks / membership testing

Lines > 90 chars have ben wrapped appropriately (except for a few cases
where readability would have decreased)

For membership testing against a container of multiple values, mostly
use `sets`!  Tuples are technically working, but semantically not the
right choice.  Tuples have structural meaning and are heterogeneous,
while sets concern membership and uniqueness, being the correct tool.

This is a similiar reasoning as in
http://stackoverflow.com/a/626871/2443886 concerning lists vs. tuples.