This PR changes all `lean-toolchain` to use relative toolchain paths
instead of `lean4` and `lean4-stage0` identifiers, which removes the
need for manually linking toolchains via Elan.
After this PR, at least Elan 4.2.0 and 0.0.224 of the Lean VS Code
extension will be needed to edit core.
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
This PR refactors `Array.qsort` to remove runtime array bounds checks,
and avoids the use of `partial`. We use the `Vector` API, along with
auto_params, to avoid having to write any proofs. The new code
benchmarks indistinguishably from the old.
