Implement memory guards in slab pool

[gavv]

core::Pool implements slab pool. (Actual implementation is in core::PoolImpl).

It already has one memory safety measure: poisoning. It fills memory with specific pattern before returning it to user and after returning it to pool.

It would be nice to add two more safety checks:

• [x] boundary (canary) guard:

  • internally, make slot bigger; insert guard areas before and after slot's memory intended for user
  • when we return memory to user, fill guard areas with some pattern
  • when user returns memory to pool, check if guard areas are still filled with same pattern; if not, issue a panic (it means that the user has buffer overflow bug)

  (pattern for memory guards should be handled by core::PoisonOps; we should add a new pattern for guard areas)

• [x] ownership guard:

  • in addition to memory guard, insert ownership field before slot's memory; it is just a pointer to the pool that owns the slot
  • when we return memory to user, set pointer to the owning pool
  • when user returns memory to pool, check if we're the owning pool; if not, issue a panic (it means that the user tries to return memory to wrong pool or passed invalid pointer)

[nolan-veed]

I'll work on this over the next few days and create 2 pull requests for it.

[gavv]

You're welcome, thanks!

[gavv]

PRs merged.