Swift multihash reimplementation/update

[cloutiertyler]

The current SwiftMultihash implementation looks to be out of date (e.g. no varint encoding). It also doesn't seem to use the Swift Package Manager.

I actually had started writing my own over at https://github.com/cloutiertyler/swift-multihash before I found that one. I've tried to use the most idiomatic Swift possible. I will be actively developing mine further and I was hoping I might be able to have it moved under multiformats if it makes sense.

I'll be adding more test cases and a README.md tomorrow.

[cloutiertyler]

I've also implemented https://github.com/cloutiertyler/swift-multibase as well.