Standard optimization: O(1) dispatch using simple-vector when all branches are constants

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If all branches can be determined to be constants at compile-time, then we can just eval all of them into a static simple-vector at compile-time, thus getting O(1) dispatch on all implementations.

There should also be a way to use a similar strategy if not all the branches are constant, but a certain threshold of the branches are (75%?). In that case, we do the O(1) dispatch into the simple-vector, and we get a distinguished value (e.g. a special private symbol) if that branch is not a constant, and then we proceed to distinguish among the non-constant cases with the main strategy.