Add `Memory.diff_*(other)`

[TexZK]

Iterates over the different stuff between a memory and something else.

diff_keys(): iterates over different addresses, one by one.
diff_values(): iterates over different values, one by one.
diff_items(): iterates over different key and value pairs, one by one.
diff_intervals(): iterates over different intervals.
diff_gaps(): iterates over different gaps.
diff_blocks(): iterates over different blocks.

More: add ^ and ^= operators to work like diff_items() (API closer to dict's operator |).
(TODO: Open dedicated issue tickets)