Per-constructor @unboxed is not compared across interfaces or coercions #9038

Whether a constructor is @unboxed (untagged) is not compared across interfaces or in coercions:

module M: {
  type t = | @unboxed A(int) | B({x: int})
  let a: t
} = {
  type t = | A(int) | B({x: int})
  let a = A(3)
}
let n = switch M.a { | A(i) => i | B(_) => -1 }   // -1: the client expects 3, the implementation built {TAG: "A", _0: 3}

The reverse direction is accepted too. So is a coercion between the two representations:

type a = | @unboxed A(int) | B({x: int})
type b = | A(int) | B({x: int}) | C
let x: b = (A(3): a :> b)   // 3 is not a b: matching it gives C

Also, when the @unboxed configurations of two variants differ, the coercion error says the wrong variant is unboxed ("Variant c is unboxed" for the plain one).

Observed on master (d48d577). Cause:

• Includecore.compare_variants compares tags of tagged constructors only, so a tagged and an untagged constructor agree.
• The coercion check compares only the declared @as (Variant_coercion.variant_representation_matches).
• Printtyp.print_variant_configuration_issue inverts "not".

Expected: an untagged constructor agrees only with an untagged one, in interfaces and coercions alike, and the error names the unboxed variant.

Found by a review of the untagged variant work (#8950).