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.

compiler/ast.nim

@@ -491,7 +491,7 @@ type
     mAnd, mOr,
     mImplies, mIff, mExists, mForall, mOld,
     mEqStr, mLeStr, mLtStr,
-    mEqSet, mLeSet, mLtSet, mMulSet, mPlusSet, mMinusSet,
+    mEqSet, mLeSet, mLtSet, mMulSet, mPlusSet, mMinusSet, mXorSet,
     mConStrStr, mSlice,
     mDotDot, # this one is only necessary to give nice compile time warnings
     mFields, mFieldPairs, mOmpParFor,
@@ -559,7 +559,7 @@ const
     mStrToStr, mEnumToStr,
     mAnd, mOr,
     mEqStr, mLeStr, mLtStr,
-    mEqSet, mLeSet, mLtSet, mMulSet, mPlusSet, mMinusSet,
+    mEqSet, mLeSet, mLtSet, mMulSet, mPlusSet, mMinusSet, mXorSet,
     mConStrStr, mAppendStrCh, mAppendStrStr, mAppendSeqElem,
     mInSet, mRepr, mOpenArrayToSeq}