Extend Item to add noise for saving/loading

[toml01]

FuzzyItem is basically a wrapper around Item.
Simply put, it is intended to obfuscate whether the value was changed during contract execution.

Currently, fuzz is implemented naively as a wrapping u8, bringing some basic obfuscation while keeping gas usage to a minimum. To improve the obfuscation quality, we can change fuzz to be either a

1. bigger integer (e.g. u32)
2. random byte(s)

at the expense of gas usage.

Another limitation is that since we have to keep a reference to Storage (to use a dtor), we have to implement both mutable and non-mutable versions e.g. FuzzyItemMut + FuzzyItem for it to be supported in both handle() and query().

Future PR would be to add an into_fuzzy_item() and into_fuzzy_item_mut() function to Item.