Construct Service with arguments in contrib/associate
Tidy up XRI normalization.

- collapse chain-called functions into a single urlescape().
- don't bother with prefixes as they are never used within the library
Merge trivial XRI tests in a single test case.
Replace use of codecutil with urllib.parse.quote in xri.py
Rename XRI_AUTHORITIES → AUTHORITIES.
xri.unprefix
Standardize on using un-prefixed XRIs.
Simplify rootAuthority
Simplify providerIsAuthoritative
Pythonic renames in xri.py
Convert CanonicalID test to use support.gentests.
Reshuffle test_xrds.CanonicalID for readability.