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/operations/ecma-comparison.c

@@ -92,9 +92,9 @@ ecma_op_abstract_equality_compare (ecma_value_t x, /**< first operand */
     }
 
     /* Swap values. */
-    ecma_value_t tmp = x;
-    x = y;
-    y = tmp;
+    x ^= y;
+    y ^= x;
+    x ^= y;
   }
 
   if (ecma_is_value_string (x))
@@ -127,9 +127,9 @@ ecma_op_abstract_equality_compare (ecma_value_t x, /**< first operand */
     }
 
     /* Swap values. */
-    ecma_value_t tmp = x;
-    x = y;
-    y = tmp;
+    x ^= y;
+    y ^= x;
+    x ^= y;
   }
 
   if (ecma_is_value_boolean (y))
@@ -192,7 +192,10 @@ ecma_op_abstract_equality_compare (ecma_value_t x, /**< first operand */
       return ecma_make_boolean_value (ecma_bigint_is_equal_to_number (x, ecma_get_number_from_value (y)));
     }
 
-    return ECMA_VALUE_FALSE;
+    /* Swap values. */
+    x ^= y;
+    y ^= x;
+    x ^= y;
   }
 #endif /* ENABLED (JERRY_BUILTIN_BIGINT) */