Path filter

Get the path filter working.

* You should be able to `make` in test_basic and do `path:` queries. You'll get 500s for search strings without a 3-char literal text run; that'll be caught and displayed better later.
* All the globbing supported by fnmatch works great.
* All of test_basic passes except test_structural_and, which awaits a working `function:` filter.