By using char const * instead of std::string we avoid significant
copying per assertion. In a simple loop with 10000000 CHECKS on
my system, this reduces the run time from 9.8s to 5.8s.
This change is at the expense of no longer capturing the second arg,
which no currently existing test notices.
In reality, this is a relatively small performance improvement,
especially with the previous improvements removing lots of superfluous
string handling, but still was measurable.
