Bug: Correctly typing and initialising prior_loops_cache

[PetarMax]

If an APR proof on disk contains a loops_cache, it cannot be executed further because of a typing error in the style of:

...
File "/nix/store/6bi78qgqbnqryswaly3m4rggsqkrj72w-python3.10-kframework-7.1.145/lib/python3.10/site-packages/pyk/proof/reachability.py", line 793, in step_proof                                                                                    
    prior_loops = step.prior_loops_cache[node.id] + (node.id,)                                                                                                                                                                                        
TypeError: can only concatenate list (not "tuple") to list     

when initialising prior_loops_cache from disk.

This PR corrects this bug by enforcing the correct types.

[tothtamas28]

Condiser pushing the conversion down into APRPRoof.__init__:

k/pyk/src/pyk/proof/reachability.py

Line 151 in 08a32de

self.prior_loops_cache = prior_loops_cache if prior_loops_cache is not None else {}