Fix printing of stats when aborted. (#6362)

Fix printing of stats when aborted. (#6362)

This PR improves/fixes printing of statistics when the solver has been aborted, i.e. when we use printSafe(), and a few other minor issues with the new statistics setup.

add toString() methods for TypeConstant, api::Kind to allow for automatic printing by print_safe<>()
improve kindToString() to avoid std::stringstream
fix newlines between statistics in printSafe()
make printing of histograms consistent
make --stats-all, --stats-expert and --stats-every-check automatically enable --stats (and vice versa)