Tracking issue for MPK

This issue tracks items remaining from adding MPK support in #7072:

- [x] fix tests when MPK is enabled (e.g., some tests of instance limits change due to striping)
- [x] add WebAssembly-level tests checking that MPK-protected slots cannot access OOB regions
- [x] enable MPK in fuzz targets
- [x] add example showing how many memories can be pool-allocated with the feature on versus off
- [x] fix up the MPK and `memory_pool` documentation; e.g., remove duplication
- [ ] handle protection key changes across asynchronous `block_on` changes (see `store.rs:1947`)
- [ ] figure out a way to run with MPK enabled in CI (QEMU?)
- [ ] benchmark `call` performance with PKRU switch

There may be others but these are the ones I could think of that would be needed before configuring the feature on by default.