Merge SourceCache into Collection

[dhardy]

INCOMPLETE: fontique tests pass; parley needs fixes

FontContext is literally just a Collection and a SourceCache. This collection already contains a QueryState, while performing a query also requires a SourceCache, so I don't see a good reason not to do this.

Motivation: I am transitioning kas-text to use fontique and will likely keep this as the primary text library in Kas since it integrates better with the widget lifecycle model. Parley is more capable, so makes more sense where complex text layout is required. I have a local hack adding optional Parley support to Kas, but this currently requires multiple instances of the Collection + SourceCache.

Simpler solution: move FontContext to fontique.

Note also: Collection and SourceCache have some option for internal sharing of state.