Add Dict.has & improve Dict helpers performance

lib/es6/Stdlib_Dict.js

@@ -10,7 +10,7 @@ function forEach(dict, f) {
 }
 
 function forEachWithKey(dict, f) {
-  Object.entries(dict).forEach(param => f(param[1], param[0]));
+  Object.keys(dict).forEach(key => f(dict[key], key));
 }
 
 function mapValues(dict, f) {

lib/js/Stdlib_Dict.js

@@ -10,7 +10,7 @@ function forEach(dict, f) {
 }
 
 function forEachWithKey(dict, f) {
-  Object.entries(dict).forEach(param => f(param[1], param[0]));
+  Object.keys(dict).forEach(key => f(dict[key], key));
 }
 
 function mapValues(dict, f) {

runtime/Stdlib_Dict.res

@@ -29,7 +29,7 @@ let forEach = (dict, f) => {
 }
 
 let forEachWithKey = (dict, f) => {
-  dict->toArray->Stdlib_Array.forEach(((key, value)) => f(value, key))
+  dict->keysToArray->Stdlib_Array.forEach(key => f(dict->getUnsafe(key), key))
 }
 
 let mapValues = (dict, f) => {