The template resolver is supposed to determine the handler, format, and
variant from the path in extract_handler_and_format_and_variant.

Previously this behaviour was close but didn't exactly match the
behaviour of finding templates, and in some cases (particularly with
handlers or formats missing) would return incorrect results.

This commit introduces Resolver::PathParser, a class which should be
able to accurately from any path inside a view directory be able to tell
us exactly the prefix, partial, variant, locale, format, variant, and
handler of that template.

This works by building a building a regexp from the known handlers and
file types. This requires that any resolvers have their cache cleared
when new handlers or types are registered (this was already somewhat the
requirement, since resolver lookups are cached, but this makes it
necessary in more situations).