Implement for[in/of]-let construct. (#3294)

This patch implements let/const support for all "for" statements.
It includes an algorithm for cloning declarative lexical environments.

JerryScript-DCO-1.0-Signed-off-by: Zoltan Herczeg zherczeg.u-szeged@partner.samsung.com

tests/jerry/es2015/for-let.js

@@ -0,0 +1,105 @@
+// Copyright JS Foundation and other contributors, http://js.foundation
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+var arr = [];
+for (let i = 0, j = 0; i < 10; j++)
+{
+  arr[i] = function() { return i + j; }
+  i++;
+}
+
+for (let i = 0; i < 10; i++)
+  assert(arr[i]() === (i * 2 + 1));
+
+var j = 0, k = 0;
+for (let i = 0; j = i, i < 10; i++)
+{
+  let i = -3;
+  assert(i === -3);
+  assert(j === k);
+  k++;
+}
+
+var j = 0, k = 0;
+for (let i = 0; eval("j = i"), i < 10; i++)
+{
+  let i = -3;
+  assert(i === -3);
+  assert(j === k);
+  k++;
+}
+
+var arr = [];
+for (let i in { x:1, y:1, z:1 })
+{
+  let str = "P";
+  arr.push(function () { return str + i; });
+}
+
+assert(arr[0]() === "Px");
+assert(arr[1]() === "Py");
+assert(arr[2]() === "Pz");
+
+try {
+  for (let i in (function() { return i; })()) {}
+  assert(false);
+} catch (e) {
+  assert(e instanceof ReferenceError);
+}
+
+try {
+  for (let i = 0, j = 0; i < 5; i++, j++)
+  {
+    if (i === 3)
+    {
+      eval("throw -42")
+    }
+  }
+  assert(false);
+} catch (e) {
+  assert(e === -42);
+}
+
+exit: {
+  for (let i = 0, j = 0; i < 5; i++, j++)
+  {
+    if (eval("i === 3")) {
+      assert(i === 3);
+      break exit;
+    }
+  }
+  assert(false);
+}
+
+var f = null, g = null, h = null;
+
+for (let i = 0;
+     f = function() { return i }, i < 1;
+     i++, g = function() { return i })
+{
+  h = function() { return i };
+}
+assert(f() === 1);
+assert(g() === 1);
+assert(h() === 0);
+
+var arr = [];
+for (const i in { aa:4, bb:5, cc:6 })
+{
+  arr.push(function () { return i });
+}
+
+assert(arr[0]() === "aa");
+assert(arr[1]() === "bb");
+assert(arr[2]() === "cc");