[PoC] Allowing ad hoc aliasing of an or pattern of literals into a polyvariant

[zth]

Now that the runtime represenation of polyvariants is stable and well defined, we can allow aliasing or patterns of literals into a polyvariant if we want, since we can guarantee that if it matches it has been refined to the provided runtime representation.

Example:

type abc = [#a | #b | #c]

let useABC = (x: abc) =>
  switch x {
  | #a => 1
  | #b => 2
  | #c => 3
  }

let fromString = s =>
  switch s {
  | ("a" | "b" | "c") as #...f => useABC(f) // `f` will be a polyvariant of the literals matched on in the pattern. So `[#a | #b | #c]`.
  | _ => 0
  }

Notes

For the implementation, we might need to check extra that the literals are valid as polyvariant constructors.

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