⚡ Make SequenceSet#xor ~1.4x faster

The results are highly dependant on data, YJIT, etc... but in my tests
this implementation seems to _consistently_ run more than 1.3x faster.

```
current (L - R) | (R - L):       905.0 i/s
0.5.8   (L | R) - (R & L):       624.1 i/s - 1.45x  slower
```

Author: nevans

lib/net/imap/sequence_set.rb

@@ -1475,8 +1475,9 @@ def intersect!(other) # :nodoc:
       # TODO: document and directly test this
       def xor!(other) # :nodoc:
         modifying!
-        both = self & other
-        merge(other).subtract(both)
+        copy  = dup
+        other = SequenceSet.new(other)
+        merge(other).subtract(other.subtract(copy.complement!))
       end
 
       # Returns a new SequenceSet with a normalized string representation.