Handle conflicts when using @position in declared types

declare A
f : int @position(2)
g : int @position(1)
end

declare B extends A
h : int @position(3)
i : int @position(2)
end

The resulting field order may not be deterministic because of

• the clashes (2)
• the missing pos (0)

this is dangerous when used with positional constraints

B( $x, $y, $z, $w ; )

Marked as minor because of the obvious workaround