Copies data from one map to another.

@param sourceRef A reference to the map source.
@param copyRef A reference to the map copy.
@param rc The current recurse and reference counter.
@returns {Map} The reference to the map copy.
function _mapCopy(sourceRef, copyRef, rc) {
    return _instanceCopy(sourceRef, copyRef, rc, (map, val, key) => {
        map.set(key, _clone(val, rc));
    });
}