evaluator: infinite loop with structural recursion

[cuematthew]

What version of CUE are you using (cue version)?

40eed232168c38e4603d9a964a90189c17277b6d

What did you do?

_t3: {
	repeater: L={L.a}
	data: M={a: a: M}
	out: repeater & data
}
_t3.out

What did you expect to see?

It should report a structural cycle.

What did you see instead?

Both evalv2 and v3 infinite loop. v3 eats memory too, so eventually this crashes with OOM. v2 doesn't seem to eat any memory at all.