Use for..of Object.keys instead of for..in (#9518)

In https://github.com/babel/babel/issues/9511 (and #9495 is another symptom), @PavelKastornyy reported a node crash becaue the JavaScript heap run out of memory. The problem was that their code was adding enumerable properties to `Object.prototype`: it is something that shouldn't be done, but Babel shouldn't make node crash if someone adds them.
I reduced down the problem to `for...in` loops in `@babel/traverse` that grew the memory consumption exponentially because of that unexpected properties.

packages/babel-plugin-transform-destructuring/src/index.js

@@ -421,7 +421,7 @@ export default declare((api, options) => {
 
         const specifiers = [];
 
-        for (const name in path.getOuterBindingIdentifiers(path)) {
+        for (const name of Object.keys(path.getOuterBindingIdentifiers(path))) {
           specifiers.push(
             t.exportSpecifier(t.identifier(name), t.identifier(name)),
           );