symmetric difference operation for sets via xor

[metagn]

closes nim-lang/RFCs#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. An operator version -+- and an in-place version like toggle as described in the RFC are also added, implemented as trivial sugar.

[mrgaturus]

if you don't want add -+- and toggle to system, i think symmetricDifference should be in setutils instead and also add -+- and toggle to that module

[metagn]

Either way is fine but: incl and excl with 2 set arguments are also simple sugar and still in the system module, and symmetricDifference is a compiler magic same as *, + etc above it; so they could all still go in system. I just figured this PR would be simpler if it just had symmetricDifference as the compiler magic. We can even use another name than -+- if we also add it to hash sets.

[Araq]

IIRC I removed the -+- operator before version 1.0 as nobody used it. Now apparently there is some demand for it, but it's so rare that it shouldn't be in system.

[metagn]

Moved to setutils

[github-actions]

Thanks for your hard work on this PR!
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