Fix not playing audio on iOS for some local paths

[t-unit]

When handing local system paths, a special constructor must be called on URL. See the official documentation.

For example, without this fix
/Users/some-user/Library/Developer/CoreSimulator/Devices/828006A2-398A-40E6-B8C4-10D6974A84EA/data/Containers/Data/Application/005825E6-2EC9-45BD-9F9F-54EE3D3033D8/Library/Application Support/audio/some-audio-file

would result in nil being handed as a URL to AVAudioPlayer.

Unfortunately the project did not compile, so I applied some quick fixes as a second commit to this PR to make the project work.
The actual fix can be found in this commit

[bsutton]

@t-unit thanks for the PR.

Just a few comments on the state of play.

I'm currently on leave and renovating my house.

We are currently trying to remove all of the swift/obj-c code from the project by using ffi.
We have made fair progress on this but really need some help from someone familiar with swift/obc/ios.

I wondering if you would be interested in getting involved in the ffi port?

[t-unit]

@bsutton sorry, can't help you at the moment with any coding support.