Make types closer to semantics

[IFcoltransG]

Review of the types that are deserialized from JSON. Adjusted to be closer to Ink runtime's actual semantics.

• Some fields of structs were Option but they weren't optional (an artefact of the port from C# nullable types?). This change required changing some constructors.
• There wasn't any 'parent' enum variant for path fragments. Change allows cleanly matching on the fragment type, rather than comparing a string for equality to "^". (An explicit todo!() has been added to resolve_path, previously ^ would break anyway, because I think resolving parent not implemented anyway.)
• Since original port was made, directional Glue has been removed from Ink, no longer G< and G>, just <>.

I also deleted unnecessary refs and borrows from the functions I was already changing — they aren't idiomatic Rust anymore.

[juniperparsnips]

There's a few compiler and test errors with the updated external args, but otherwise everything looks good and we can merge after they're fixed

[IFcoltransG]

That's embarrassing on my part. I've corrected the test cases that don't compile. One test case fails, which is the same one from #10.