Improve BigInt support (#4118)

This patch adds several small features:
- Support right shift with negative numbers
- Support exponentation operator
- BigInts can be enabled in ES5.1 mode
- Remove dead code from ecma_deref_bigint
- Support longer BigInt literals in the parser
- Fix various BigInt comparison issues
- Do not discard unary plus for BigInt constants

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

jerry-core/ecma/builtin-objects/ecma-builtins.inc.h

@@ -678,6 +678,8 @@ BUILTIN (ECMA_BUILTIN_ID_MAP_ITERATOR_PROTOTYPE,
          map_iterator_prototype)
 #endif /* ENABLED (JERRY_BUILTIN_SET) */
 
+#endif /* ENABLED (JERRY_ESNEXT) */
+
 #if ENABLED (JERRY_BUILTIN_BIGINT)
 /* The %BigInt.prototype% object */
 BUILTIN (ECMA_BUILTIN_ID_BIGINT_PROTOTYPE,
@@ -694,8 +696,6 @@ BUILTIN_ROUTINE (ECMA_BUILTIN_ID_BIGINT,
                  bigint)
 #endif /* ENABLED (JERRY_BUILTIN_BIGINT) */
 
-#endif /* ENABLED (JERRY_ESNEXT) */
-
 #if ENABLED (JERRY_BUILTIN_DATAVIEW)
 /* The DataView prototype object (ECMA-262 v6, 24.2.3.1) */
 BUILTIN (ECMA_BUILTIN_ID_DATAVIEW_PROTOTYPE,