This PR improves the resilience of the cached member fields in Circuit. Most importantly, _placement_cache, which is used to speed up Circuit.append() from O(N) to O(1), is made to be resilient and consistent across insert and batch inserts, (削除) freeze/unfreeze cycles (削除ここまで), add and mul operations, and copy. Prior to this PR, performing any of those actions on a circuit would delete the cache, and append would fall back to O(N) behavior from that point on.

Summary of changes:

• _PlacementCache has new methods insert_moment(index, count) and put(index, mop), that update the key indices to account for new moments or operations. Circuit has new corresponding _insert_moments(...) and _put_ops(...) methods to modify the circuit and update the cache atomically, and logic strewn around Circuit has been updated to use those methods when possible.
• Circuit.copy() now preserves all cache values.
• All cached values except _placement_cache are now immutable types, so copy doesn't have to make copies.
• A simple optimization to Moment.with_operations(*ops) gave a 15% speedup in the common case of len(ops)==1.

Overall, the performance of append and insert individually are unaffected by this change; measured time is equal, but the performance when mixing inserts and appends is improved from O(N) to O(1). It also cleans up the code a bit IMO, with less need to place self._mutated() everywhere.

(削除) The biggest open question I have is whether building the placement cache in Circuit.from_moments() justifies the extra latency. If not, that could be removed, and it would only affect the freeze/unfreeze cycle case. (削除ここまで) Update: it does not, and that functionality has been reverted.

Fixes #7464