MapConstraint of MapConstraint

[QuentinRoy]

It would be nice to be able to create a MapConstraint of a MapConstraint..

For example:

var cjsMap = cjs({
    '1': 'a',
    '2': 'b'
});
var other = cjs(cjsMap);

console.log(cjsMap.item('1')); // a
console.log(other.item('1')); // undefined :(