Another variant of 51, moving the embedding out to an implicit
unification.

See also 55.

-- a.cue --
d: x: 5
a: d
b: a
b: {
	x: int
	c: x
}
-- out/trim --
== a.cue
d: x: 5
a: d
b: a
b: {
	x: int
	c: x
}
