Swift 5 migration

[GeroHerkenrath]

It seems there wasn't much to do to migrate this to Swift 5, but since this is the first time I propose a PR to a foreign repo I am not sure whether I perhaps forgot something.

Besides the normal project file updates I would also propose this to increase the version of the pod to 4.1 as I am not sure it is fully backwards compatible. Thus I changed that in the podspec (together with specifying the swift version there instead of a .swift-version file).

[GeroHerkenrath]

Well, I guess that's as far as I get for today. Seems like that needs to be fixed in Embassy first.

A quick google revealed that the problem with String(cString: UnsafePointer(strerror(errno))) can be relatively easy to fix: https://forums.developer.apple.com/thread/113919

This will probably also be relevant for envoy/Embassy#70, I might join in on that PR as well

[GeroHerkenrath]

In case the thread in the Apple dev forums is lost, it should be noted that according to the answer there it would be possible to rewrite String(cString: UnsafePointer(strerror(errno))) as String(cString: strerror(errno)) since there is a specific overload in String to work with the c string directly.

[LFabien]

String(cString: UnsafePointer(strerror(errno))) was rewritten String(cString: strerror(errno)) here:
envoy/Embassy#66

Thanks for the PR @GeroHerkenrath !