Converted SHA checksum code to Swift & added Swift Package Manager support

[alexeichhorn]

Hi,

I converted the last bit of code that was still written in Objective-C to Swift. It was the implementation of the different SHA checksum algorithms (as suggested in #167 ). This made it much easier to add support for the new Swift Package Manager, which was my actual plan for this contribution.

Happy to see your feedback on this.

[bonkowski]

Any plans to accept this PR?

[pc-scoop]

this is taken care in https://github.com/TakeScoop/SwiftyRSA/pull/231