This PR makes functions defined by well-founded recursion use an
`opaque` well-founded proof by default. This reliably prevents kernel
reduction of such definitions and proofs, which tends to be
prohibitively slow (fixes#2171), and which regularly causes
hard-to-debug kernel type-checking failures. This changes renders
`unseal` ineffective for such definitions. To avoid the opaque proof,
annotate the function definition with `@[semireducible]`.
This PR takes Array-specific lemmas at the end of `Array/Lemmas.lean`
(i.e. material that does not have exact correspondences with
`List/Lemmas.lean`) and moves them to more appropriate homes. More to
come.
