This PR removes support for reducible well-founded recursion, a Breaking
Change. Using `@[semireducible]` on a definition by well-founded
recursion prints a warning that this is no longer effective.
With the upcoming module system, proofs are often not available. With
this change, we remove a fringe use case hat may require proofs, and
that would not be supported under the module system anyways.
At least for now, direct use of `WellFounded.fix` is not affected.
This fixes: #5192
This PR replaces `Array.feraseIdx` and `Array.insertAt` with
`Array.eraseIdx` and `Array.insertIdx`, both of which take a `Nat`
argument and a tactic-provided proof that it is in bounds. We also have
`eraseIdxIfInBounds` and `insertIdxIfInBounds` which are noops if the
index is out of bounds. We also provide a `Fin` valued version of
`Array.findIdx?`. Together, these quite ergonomically improve the array
indexing safety at a number of places in the compiler/elaborator.
Previously, it was not possible to use `decide` with most Array
functions (including `==`).
Later, we may replace some of these functions with defeqs that go via
the `List` operations, and use `csimp` lemmas for fast runtime
behaviour. In the meantime, this allows using `decide`.
