Ensure chunked(by:) always compares consecutive elements

[timvermeulen]

Fixes #161.

[a, b, c, d].chunked(by: { print($0, $1); return true })
// previous behavior: (a, b), (a, c), (a, d)
// correct behavior:  (a, b), (b, c), (c, d)

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

[timvermeulen]

@swift-ci Please test