Implement the core of let/const statement. (#3239)

This patch implements the core part of let/const statements. Redeclarations are correctly
detected and separate contexts are correctly created for these statements. Register
optimizations are also emplyed whenever possible.

Lots of features are still missing:
 - checking the var statements in eval
 - const are treated as lets
 - single statement checks are missing
 - export declarations are exported as vars, let/const export is not supported

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

tests/jerry/func-decl.js

@@ -16,7 +16,7 @@ function f() {
     return 'foo';
 }
 assert ((function() {
-    if (1 === 0) {
+    if (1 === 1) {
         function f() {
             return 'bar';
         }