LibJS: Integrate iterator protocol into language features

Finally use Symbol.iterator protocol in language features :) currently
only used in for-of loops and spread expressions, but will have more
uses later (Maps, Sets, Array.from, etc).

Libraries/LibJS/Tests/loops/for-of-basic.js

@@ -28,6 +28,59 @@ describe("correct behavior", () => {
         for (char of "abc");
         expect(char).toBe("c");
     });
+
+    test("respects custom Symbol.iterator method", () => {
+        const o = {
+            [Symbol.iterator]() {
+                return {
+                    i: 0,
+                    next() {
+                        if (this.i++ == 3) {
+                            return { done: true };
+                        }
+                        return { value: this.i, done: false };
+                    },
+                };
+            },
+        };
+
+        const a = [];
+        for (const k of o) {
+            a.push(k);
+        }
+
+        expect(a).toEqual([1, 2, 3]);
+    });
+
+    test("loops through custom iterator if there is an exception thrown part way through", () => {
+        // This tests against the way custom iterators used to be implemented, where the values
+        // were all collected at once before the for-of body was executed, instead of getting
+        // the values one at a time
+        const o = {
+            [Symbol.iterator]() {
+                return {
+                    i: 0,
+                    next() {
+                        if (this.i++ === 3) {
+                            throw new Error();
+                        }
+                        return { value: this.i };
+                    },
+                };
+            },
+        };
+
+        const a = [];
+
+        try {
+            for (let k of o) {
+                a.push(k);
+            }
+            expect().fail();
+        } catch (e) {
+            expect(a).toEqual([1, 2, 3]);
+        }
+    });
 });
 
 describe("errors", () => {
@@ -35,13 +88,13 @@ describe("errors", () => {
         expect(() => {
             for (const _ of 123) {
             }
-        }).toThrowWithMessage(TypeError, "for..of right-hand side must be iterable");
+        }).toThrowWithMessage(TypeError, "123 is not iterable");
     });
 
     test("right hand side is an object", () => {
         expect(() => {
             for (const _ of { foo: 1, bar: 2 }) {
             }
-        }).toThrowWithMessage(TypeError, "for..of right-hand side must be iterable");
+        }).toThrowWithMessage(TypeError, "[object Object] is not iterable");
     });
 });