sub methods.

Currently with even more code repetition that needs to be factored out.

files generated for testing.

Still with plenty of repeated code to be factored out.

The implementation is not trivial, and I'm still not entirely clear
on the use case for Varnish/VCL. We probably will need partial
matching to implement matches against the request body.

so that backref/namedref are guaranteed to return strings in workspace.

There's still a bit of code repetition to be factored out.

for use by the backref/namedref functions.

This brings about a major refactoring, to factor out common code for
regex compiles and matches.

failure messages.

after using pcre2_match_data_create_from_pattern() to create match_data.

This requires a test in configure.ac for whether UTF8 is enabled in
the PCRE2 lib against which the VMOD is built.

and recursion_limit, and test compile option use_offset_limit

note that I don't know at the moment how to test never_backslash_c