Remove the inout on reductions(into:_:) to be consistent with the stdlib reduce(into:_)

[kylemacomber]

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[kylemacomber]

@swift-ci please test

[kylemacomber]

@swift-ci please test

[kylemacomber]

cc @danielctull

[danielctull]

Thanks for fixing that. I’m not sure where the idea that it should be inout came from! 🙂