Refactor canonicalization more

[srawlins]

A few tidyings:

• Canonicalization.scoreCanonicalCandidates is now calculateCanonicalCandidate, and returns one Library. ScoredCandidate is then made private. Stop recalculating ElementModel.locationPieces many times to determine a canonical library.
• Mixin hierarchies changed to support this, and I think are more correct: Canonicalization now implements Warnable; Warnable no longer implements Canonicalization; Warnable now implements Documentable and Locatable; Documentable is now a mixin; DocumentationComment now subtypes Canonicalization.
• Many assert(x != null)s are removed as they are duplicate with another assert that immediately follows.

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