fix(library/init/wf): `well_founded.recursion` should be a `def` (#503)
fix(library/init/wf): `well_founded.recursion` should be a `def` (#503)

This declaration returns a `Sort` but is defined as a `lemma`, which causes "cannot generate vm code" errors when you try to use it. And also mathlib's linter doesn't like it.

Zulip thread: https://leanprover.zulipchat.com/#narrow/stream/113488-general/topic/Sholdn't.20.60well_founded.2Erecursion.60.20be.20a.20.60def.60.3F



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