Add an unsafe method that allows the user to recover the allocator from the memory.

[Determinant]

Hi team, we're a startup working on a platform which relies on a software-defined persistent (non-volatile) memory. So that means a program (in our case a piece of WASM code) always has its memory persisted. Talc currently is a perfect fit because it is lightweight and can help us manage the allocation for these "magical" variables. But in order to get this to work, we added a method called recover_from_existing_heap that reconstructs the state of Talc based on the same span of the memory it previously worked on.

We feel this could be an interesting and general feature to talc, especially for the non-volatile memory use case. Would like to see if you're interested in merging the code.