Patch release: no API or behavior change. Pins a Minimum Supported Rust
Version and guards it in CI.

- Declare rust-version = "1.78".
- Revert the two debug_assert uses of <int>::is_multiple_of (stabilized in
  1.87) to `x % n == 0`.
- Enable clippy::incompatible_msrv.
- Add a CI `msrv` job.