REF: use CoW helpers more
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@phofl in Block._maybe_copy there is a path where we can pass deep=False to Block.copy.  i think we could simplify things if instead we just returned `self` there.  would that be OK? I’ll have to look into this more, there are scenarios where this could be problematic. This ties a bit into my todo list that’s related to replace. Might get to this over the weekend or next week  > there are scenarios where this could be problematic

Yah, just remembered there are a couple places where we set `block.values` post-`__init__`.  Refactoring to avoid those might make that safer.  I don't think this PR should be affected though. i think the helper here should be OK?