018 destructure effect branch

// Destructure on an EFFECT branch — handlers splice inside the producer's
// extent (handlers-struct path), per-field consts off the shared param.
~import std/io

~pub event scan-pairs {}
! pair { a: i64, b: i64 }
| done

~proc scan-pairs|zig {
    pair(.{ .a = 1, .b = 2 });
    pair(.{ .a = 3, .b = 4 });
    return .done;
}

~scan-pairs()
! pair { a, b } |> std/io:print.ln("{{ a:d }}+{{ b:d }}")
| done |> std/io:print.ln("done")