Should Slicts be mutable?

[maxhutch]

There is nothing intrinsic about the slice-able interface that makes it immutable. Slicts are currently immutable to avoid inconsistency in the back-end dictionary. Slicts store references to the dictionary, not the (key, value)s themselves, so operations on Slicts would be seen by all other Slicts of the same dictionary.

Adding items directly to CachedSlicts would be more efficient than adding items to the back-end dictionary and calling update_cache.