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/common.h

@@ -74,6 +74,9 @@ typedef enum
   LEXER_FLAG_SOURCE_PTR = (1 << 2), /**< the literal is directly referenced in the source code
                                      *   (no need to allocate memory) */
   LEXER_FLAG_LATE_INIT = (1 << 3), /**< initialize this variable after the byte code is freed */
+#if ENABLED (JERRY_ES2015)
+  LEXER_FLAG_GLOBAL = (1 << 4), /**< this local identifier is not a let or const declaration */
+#endif /* ENABLED (JERRY_ES2015) */
 } lexer_literal_status_flags_t;
 
 /**