swiftup: the Swift toolchain installer

HI!
Its an actually Swift version of swiftenv(https://github.com/kylef/swiftenv\).
It now can install multiple versions and have the basic facilities of a
version manager. Though it is not complete and there are many to improve
but worth a try. It only works on Linux for now. It was a project to learn
Swift so code qualities are not soo good :)

PR welcome!

Check it out and feedback!

Thank you all!