Fix the deadlock issue on iOS 11

[ameyjain]

If the swift version is less than 5.0 instead of calling the default function: https://developer.apple.com/documentation/swift/2632391-withunsafemutablebytes
it calls itself causing a deadlock.

[jagdeep-manik]

+1, I ran into the same issue. It'll cause an infinite loop. @daltoniam

As an aside, the way swift package manager is set up makes it so that it builds using swift 4.2 even though we could technically support swift 5. I wish there was a way to tell SPM to build with this with swift 5. 😬

I'm also wondering if we need those version checks in the first place--it looks like all those functions in those helpers are available in swift 4. Does swift 5 require some different memory management that forces us to have special handling?

[amayers]

The reason that SPM is building it with 4.2, is because this line at the top of Package.swift.

// swift-tools-version:4.2

I just opened #797 to update it to 5.2, since if you install via Cocoapods it will be using 5.0, and everything builds/tests fine on 5.2.

[daltoniam]

Thanks! Appreciate the PR!