improve reference count validation

[hawkw]

• fix: incorrect wrapping when overflowing maximum ref count

  Currently, when the maximum reference count for a slot is reached, the reference count wraps to 0. This is wrong, as it could potentially cause a use-after-free by allowing the slot to be released while there are still active references.

  This commit fixes this by adding a check in Slot::get that creating a new reference would not overflow the reference count. If the reference count would overflow, get returns None instead. Additionally, I've added debug assertions that ensure this check works correctly.

  I've also added a new test for this.

  Fixes #22

• feat(Config): validate concurrent refs

  This commit adds a check in Config::validate that there can be at least 2 concurrent references to a slot. Otherwise...what's the point?

  Fixes #21

[hawkw]

cc @tmandry