These methods force a `MapObserver` to own an initial value, but
there's no reason for this to be the case - If you don't need to allow a
dynamically-changeable initial value, it might be nice to use `<<Self as
MapObserver>::Entry as Default>::default()` everywhere and have the compiler
statically propagate that value.
Not a lot of code used these methods (which seems like a good argument that
they aren't a fundamental part of the inteface).
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