make xcframework done manually by PR authors is error-prone and a potential security hazard

[daira]

Ideally make xcframework would be automatic. Having it be done manually by the committer is error-prone and results in PRs that can't be fully reviewed, which is a security hazard (for example if the committer's machine / build environment is compromised).

If make xcframework is deterministic (I don't know whether it is or could be made so), then perhaps a CI step could do make clean xcframework and then check that the resulting tree is clean.

Originally posted by @daira in #98 (comment)

[daira]

@nuttycom wrote:

I think that we need to overhaul how these build artifacts are stored as well; having them committed as blobs to github is really not ideal, because the repo size is enormous. When we fix the blob storage problem, we'll probably put the FFI layer back into ZcashLightClientKit as well.

[nuttycom]

Related to the comment of mine that @daira mentioned, this repository should no longer be maintained under zcash-hackworks in any case. I think that the best way forward here is:

1. Move the source code into the [zcash/ZcashLightClientKit] repository (for consistency, it would also be good to rename that repository zcash-ios-wallet-sdk for consistency with the Android repository, but that's a separate niggle.)
2. Set up automated builds that publish to an object store as a CI action.