Copies data from one item to another.
Designed to a be a generic copy interface, where possible.

@param sourceRef A reference to the source.
@param copyRef A reference to the copy.
@param rc The current recurse and reference counter.
@returns {any} The reference to the copy.
function _singleCopy(sourceRef, copyRef, rc) {
    return _instanceCopy(sourceRef, copyRef, rc, (item, val, key) => {
        copyRef[key] = _clone(val, rc);
    });
}