Add request parsing and related tests

This commit adds a function for processing a “path info” string, for
example one received through C·G·I (as the `PATH_INFO´ environment
variable), into a structure which represents its semantics,
`cgirls_req´. It also adds a function for reserializing this structure
into a canonical form. The program `cgirls-test-pathinfo´ is used with
the existing test infrastructure to ensure that strings are processed
correctly.

There is a flaw in this design (which I realized after making the
original commit, but before writing this updated message), in that an
empty identifier string is represented as `..´, which in a URL already
has a different, and very normative, meaning of “parent directory”.
This flaw will need to be fixed in a later commit.

Probably some more tests could be added here; in particular only a few
verbs and extensions are being tested right now and ideally they all
would be.