rename files

remove String::at : ES7 collision, implement later
add static Array.{keys, values, entries}
src/symbol.js -> src/es6_symbol.js
src/collections.js -> src/es6_collections.js
src/iterators.js -> src/es6_iterators.js
src/promise.js -> src/es6_promise.js
src/arrayStatics.js -> src/array_statics.js
src/extendCollections.js -> src/collections.js