Canonical representation 2

[mlagally]

Chapter structure skeleton

targets to address issue #55

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[mmccool]

Probably should retire this PR, since canonicalization is now in the TD spec, but I will review first to see if any content (eg use case discussion) should be moved over to the TD spec.

[mmccool]

Should replace (if necessary) with a link to the Canonical TD definition in the TD spec. Note however that due to sorting a canonical TD may have to be loaded into memory before processing (we considered but rejected a feature to sort definitions so they appear before uses... it complicated the definition a lot).

[mmccool]

Two points discussed in this PR which are not in the TD version:

• Base URLs (not sure why this is an issue; I guess we should forbid people "expanding" all URLs. If a base URL is used in the input and relative URLs in forms, the same should be used by the output of a canonicalizer...)
• Encrypted TDs (this is orthogonal to the issue of canonicalization, but is related to signing; of course a signed TD may not be encrypted, but a public-key-encrypted TD would allow validation that it was encrypted by the owner of the private key).

[mmccool]

• Regarding "base": We could forbid it in canonical form, forcing expression of all URLs in absolute form, but that seems overly draconian and makes chaining signed changes to the base inconvenient/impossible. However I realized not allowing changes to the base and the endpoint URLs was just a special case of an assertion I already had not allowing changes to how URLs are expressed. I made this explicit here: More fixes to canonicalization wot-thing-description#1129, please review.
• Regarding encryption: I don't think this belongs under canonicalization, but rather under a discussion of signing, or possible under a discussion of object security for TDs in discovery. So I left this out of my version.

[mlagally]

vF2F on 23.6.:
Canonical forms and signing are targeted for the TD.
This PR needs to be adapted to reflect the state in the TD.
Also some parts wrt. to canonical TDs should be removed.