Replace unsafe iteration with use of Span

[fboemer]

Fixup of #242

This replaces a number of withUnsafe blocks with use of the .span property. This was introduced in Swift 6.2, so it does require bumping the toolchain version and minimum Mac deployment target.

Some minor performance regressions should be anticipated, as there will now be bounds checks performed where previously these were compiled out with the use of unsafe buffers. These either should be accepted as the cost of memory safety, or investigated to see if tweaks to the code structure could help the compiler eliminate them.

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CI / swifttests / Linux ( - - aarch64) (pull_request) CI failure seems to result from https://github.com/swiftlang/github-workflows/pull/207/files, perhaps to be fixed in swiftlang/github-workflows#210.

We also enable breaking API changes, which correctly detects:

2 breaking changes detected in HomomorphicEncryption:
  💔 API breakage: func Array2d.withUnsafeData(_:) has been removed
  💔 API breakage: func Array2d.withUnsafeMutableData(_:) has been removed

[karulont]

LGTM