* `x=*` constrained by `+x` now produces a boolean valued variant instead of a multi-valued variant.
* Values are now always stored as a tuple internally, whether bool, single or multi-valued.
* Value assignment has a stricter api to prevent ambiguity / type issues related to
`variant.value = "x"` / `variant.value = ["x"]` / `variant.value = ("x",)`. It's now `variant.set("x", ...)` for
single and multi-valued variants.
* The `_original_value` prop is dropped, since it was unused.
* The wildcard `*` is no longer a possible variant value in any type of variant, since the *parser*
deals with it and creates a variant with no values.
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