135 effect arms exclusive with resume type

// An effect's resume is EITHER a single anonymous `-> T` OR a named sum of
// arms — never both. The arms ARE the resume; a `-> T` alongside them has
// nothing left to type. Sibling of the accept pins 210_092/093.
~pub tor request { payload: i32 }
! ask i32 -> i32
    | halved i32
    | timeout
| done i32

~proc request|zig {
    const r = ask(payload);
    return .{ .done = r };
}