See #1036. Since then we have `MockExecutor` which works much better. Notabily several tests that were flaky were rewritten to use the new machinery and became reliable. We have declared `MockSleepProvider` and `MockSleepRuntime` as deprecated. I think we don't intend to fix #1036 or the other issues with `MockSleepProvider`. So we should abolish them. 1. [x] Properly deprecate them with `#[deprecated]` and appropriate `#[allow]` !2843 2. [ ] Rewrite all in-tree users to use `MockExecutor` 3. [ ] Remove them 4. [ ] Remove the `block_advance` etc. methods from `Runtime` 5. [ ] Possibly rename `SimpleMockTimeProvider` to `TimeProvider` and `simple_time` to `time`, eventually.