Persistent store

[ia0]

• Do not panic if store doesn't initialize, just fail all storage operations. Done in opt_store branch but increases binary size from 124.1k to 125.7k.
• [Blocked by Tock 2] Use read-only slices now that https://github.com/tock/tock/issues/1274 is fixed.\
• Look at fuzzing coverage, some functions (like remove_handle) or error conditions are not covered.
• Add a fuzzing mode 2 full cycles before end of lifetime.
• Update the fuzzing moder for dirty storage to start from one of the other mode, then fork and modify the storage, then continue. There's probably a way to use the fuzzer to mutate the storage.