Add Dict.mapPairs & experimental modules

Dict.mapPairs({q:1,w:2,e:3}, (v,k)=> k!=2 && [k+k, v*v]);
// => {qq:1,ee:9}

Experimental abstract references module:
https://github.com/zenparsing/es-abstract-refs
Add symbols & function, map, weakmap, dict methods

Experimental iterator module:
Play with %IteratorPrototype% & iterators chaining