When introduced the abstraction was used for mocking socket connections,
but will now also be used for communicating over stdin/stdout with the
parent- or with child processes.

Co-authored-by: Jan Beinke <git@janbeinke.com>

Due to the accepted "monad of no return" proposal, return becomes an
alias for pure. Return can be a pitfall for newcomers, so we decided to
use pure instead.

Co-authored-by: Jan Beinke <git@janbeinke.com>

Co-authored-by: Jan Beinke <git@janbeinke.com>

Co-authored-by: Jan Beinke <git@janbeinke.com>

Co-authored-by: Jan Beinke <git@janbeinke.com>