a top-level struct declaration produces a structure type entry
with each field's type interned.
the by-value dependencies are correctly added as dependencies
and contribute to the ordering.

<<<

Point = type { x int, y int }

>>>

(unit
	(types
		(type 0 int primitive)
		(type 1 uint primitive)
		(type 2 bool primitive)
		(type 3 string primitive)
		(type 4 float primitive)
		(type 5 byte primitive)
		(type 6 ascii primitive)
		(type 7 void primitive)
		(type 8 Point structure (field x (ref int)) (field y (ref int)) (depends_on 0 0)))
	(emission_order 0 1 2 3 4 5 6 7 8))