C4 linearization: C3 with extra struct (linear) inheritance constraint

[fare]

Extends the C3 algorithm with a 4th constraint, that struct subclasses can only mix if they form a linear struct inheritance chain (ie no diamonds).

This allows us to:

• lift the restriction of structs only inheriting structs; they can inherit classes so as to fix the slot layout.
• lift the single inheritance restriction of structs, which can now mix in multiple classes, subject to the linear inheritance constraint.

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[vyzo]

fixed a bunch of things and removed dead code; make-class-instance is now just make-instance and the artist formerly known as make-instance is a make-new-instance.