Quick thoughts and ideals:

- Schemes are compositional, "wrapping" super contexts: `c+b+a://a/b/c`
- The scheme communicates the target type (above is `c`)
- URIs can be arbitrarily relative so long as they're resolved in the right contexts.
  Above, `c+b://b/c` can be resolved in the context of `a://a`
- URIs are resolved by unwrapping schemes and resolving in to out
- URL params can apply only to the target type (this is the most consistent and probably
  not too restrictive)
- Trajectories from one scheme to another can be inferred from the type hierarchy; there
  may be many