Providing an accepts method for StrDistance allows removal of special casing
for the tr/// in make_smartmatch. Additionally it seems to be negligibly
faster (~.003 less D misses, according to cachegrind).
This brings us in line with S04. The new 'use fatal' implementation
does not cause action-at-a-distance. Only one spectest needed some
adaptation for this change, so hopefully fallout in the ecosystem
will be rather minimal.