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

jerry-core/parser/js/js-parser-util.c

@@ -1117,6 +1117,10 @@ parser_error_to_string (parser_error_t error) /**< error code */
     {
       return "Rest parameter may not have a default initializer.";
     }
+    case PARSER_ERR_VARIABLE_REDECLARED:
+    {
+      return "Local variable is redeclared.";
+    }
 #endif /* ENABLED (JERRY_ES2015) */
 #if ENABLED (JERRY_ES2015_MODULE_SYSTEM)
     case PARSER_ERR_FILE_NOT_FOUND: