Reduce function calls in creation of Item classes

With heavy use of Stash creation of Item classes & setKey shows up
in profile data, so the change tries to reduce function calls in these.

Suggested followup is executeGet().
Note: Might need another pass on this tomorrow for CS / tests if this is otherwise accepted.
Note2: Contains interface change on Item to force array type on $key, felt like a easy way to avoid having to check.
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