[Swift4] advanced(by:) ->index(_:, offsetBy:)

[wspl]

#56 Fixed

[sharplet]

Thanks for the contribution! This looks good to me, though I'm not sure why the one build is failing. Maybe swiftenv install --list-snapshots and swiftenv install <snapshot> disagree about what the latest snapshot actuall is? I'll do some manual testing later today if CI keeps failing.

[sharplet]

Fixes #56.

(I believe the issue number has to appear after the keyword for GitHub to automatically close the issue when this is merged.)

[sharplet]

Did some manual testing with the latest Swift 4 snapshot on Linux, found this:

/Regex/Source/Foundation+Ranges.swift:4:34: error: use of undeclared type 'TextCheckingResult'
typealias NSTextCheckingResult = TextCheckingResult
                                 ^~~~~~~~~~~~~~~~~~

Don't have time to debug this further today, hoping to get to it tomorrow.

[sharplet]

There are some unrelated issues with the latest Swift 4 snapshots, but otherwise this seems just fine, so I've merged it manually. Thanks again!