valuewrapper can be used only to generate atomic wrapper types for
types which are stored as atomic.Value. This isn't necessary because the
same logic can be applied to atomic wrappers built on other types like
Duration and Bool.
Generalize valuewrapper into atomicwrapper with support for optional
pack/unpack functions which handle conversion to/from `interface{}`.
This lets Error hook in and install the `storedError` struct (now called
`packedError`) to avoid panics from nil storage or value type change.
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