symmetric difference operation for sets via xor (#24286)

closes https://github.com/nim-lang/RFCs/issues/554

Adds a symmetric difference operation to the language bitset type. This
maps to a simple `xor` operation on the backend and thus is likely
faster than the current alternatives, namely `(a - b) + (b - a)` or `a +
b - a * b`. The compiler VM implementation of bitsets already
implemented this via `symdiffSets` but it was never used.

The standalone binary operation is added to `setutils`, named
`symmetricDifference` in line with [hash
sets](https://nim-lang.org/docs/sets.html#symmetricDifference%2CHashSet%5BA%5D%2CHashSet%5BA%5D).
An operator version `-+-` and an in-place version like `toggle` as
described in the RFC are also added, implemented as trivial sugar.

lib/system/jssys.nim

@@ -337,6 +337,18 @@ proc SetMinus(a, b: int): int {.compilerproc, asmNoStackFrame.} =
     return result;
   """.}
 
+proc SetXor(a, b: int): int {.compilerproc, asmNoStackFrame.} =
+  {.emit: """
+    var result = {};
+    for (var elem in `a`) {
+      if (!`b`[elem]) { result[elem] = true; }
+    }
+    for (var elem in `b`) {
+      if (!`a`[elem]) { result[elem] = true; }
+    }
+    return result;
+  """.}
+
 proc cmpStrings(a, b: string): int {.asmNoStackFrame, compilerproc.} =
   {.emit: """
     if (`a` == `b`) return 0;