Use an inlined helper function instead.

Also clang-format affected files.

Fixes #23

Error messages are generated by other means, so Check::Description
no longer serves a purpose.

TODO: more tests

That's handled at a higher level by making a full regex for the check line.

In a pattern, a regexp is delimited by {{ and }}

Move the prefix/substring contraint from the matches call
to a property of the string property itself.