Minimal URI escaping

[chrysn]

Warning: This is based on a not-yet-concluded specification precision suggestion. You might not want to merge this until the discussion there is concluded; this serves to illustrate, and as a reference implementation of the proposed canonicalization.

There is some over-escaping happening currently before the MD5 hashing; in particular, a colon (':') is escaped without need.

While discussion is ongoing at FDO and there may be follow-ups on the IETF side, I think that what is proposed here is what should be the canonical encoding.

Concrete changes:

• The to-be-escaped set is changed from the illegal characters of the userinfo component (of which I don't understand why it was picked for here) to the set described by the allowed characters in a path segment (named pchar in RFC3986, and imported from RFC8089 via the path-absolute rule.
• A ./ prefix is introduced iff there is a colon in the relative reference. (Otherwise URI parsing would give completely different results when parsed back).
• Tests to capture the new behavior. (The existing tests didn't even notice the changes).