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ae9287c4f3
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.