083 effect branch resume binding

// Test: effect-branch handler produces the binding itself as the resume value.
//
// The effect is declared `! squarify i32 -> i32`, so the arm produces with
// `-> n` — the bare binding `n` echoed back as the resume value. `->` is the
// produce glyph; `=>` would be illegal (the effect has no branches).

~import std/io

~pub event transform { payload: i32 }
! squarify i32 -> i32
| done i32

~proc transform|zig {
    const squared = squarify(payload * payload);
    return .{ .done = squared };
}

~transform(payload: 7)
! squarify n -> n
| done result |> std/io:print.blk {
    result is {{ result:d }}
}