Thanks to Nicolas Pfeiffer for writing the initial prototypical
implementation of continuation stealing and the cactus stack
mechanism, on which this is based.

Co-authored-by: Nicolas Pfeiffer <pfeiffer@cs.fau.de>

To skip test if emper::IO is false the new io test runner in
tests/test-runner is introduced which checks emper::IO and skips the
test otherwise executes our normal testMain function.

Fixes #10.