SwiftPM Support

[adamstener]

Closes #3

This PR adds SwiftPM support to MnemonicSwift.

Some important changes needed to happen to make this work.

Namely, we had to increase to swift 5.3 in order to allow SPM to copy resources into the testing bundle.

CryptoKit was also not supporting SPM so this PR moves us to Apples swift-crypto or "Crypto" instead of the previous CryptoKit.

All tests are passing, whether you open the project or the package, and I've cleaned up some of the various formatting issues. Happy to squash after approval if needed, I kept these as separate commits to more easily review what changed to obtain SPM support.

[pacu]

LGTM, deferring to @defuse to take a look from Sec PoV

[daira]

Swift-Crypto is Apache 2.0-licensed only. MnemonicSwift is MIT-licensed. Apache 2.0 is a permissive license and section 4 allows relicensing as long as the conditions of the Apache 2.0 license are complied with. My reading is that to be sure to have complied with clause 4 d), you should copy NOTICE.txt from Swift-Crypto into NOTICE.SwiftCrypto.txt in this repo.

I would also recommend to consider dual-licensing this repo as "MIT or Apache-2.0".

[adamstener]

Swift-Crypto is Apache 2.0-licensed only. MnemonicSwift is MIT-licensed. Apache 2.0 is a permissive license and section 4 allows relicensing as long as the conditions of the Apache 2.0 license are complied with. My reading is that to be sure to have complied with clause 4 d), you should copy NOTICE.txt from Swift-Crypto into NOTICE.SwiftCrypto.txt in this repo.

I would also recommend to consider dual-licensing this repo as "MIT or Apache-2.0".

@daira I believe this is done, can you double check that I've addressed your concerns properly though? Thanks!

[pacu]

Approved. Just waiting on licensing changes to be reviewed by @daira

[pacu]

Opened a new issue in order to not block this PR https://github.com/zcash-hackworks/MnemonicSwift/issues/12