Possible Over-Iteration in PrettyBytes

I was looking for a not `%02x` way of converting binary to hex `String`, and I stumbled upon the implementation in `PrettyBytes.swift`.

https://github.com/apple/swift-crypto/blob/9cc89f0170308b813af05dadcd26f9a2dee47713/Sources/Crypto/Util/PrettyBytes.swift#L45-L51

I'm not 100% sure about the `Sequence` conformance of non-contiguous `DataProtocol`, but it seemed to me that the `self` on line 46 should be the ignored parameter `$0` (or maybe eliminate the region logic since looping over `self` most likely would have taken non-contiguous memory regions into account?), otherwise the code reads:

> For **each segment** of the data buffer, loop over **the entirety of data buffer** and convert each byte to two hex chars.

Unless non-contiguous `DataProtocol` has non-stable indices and iterators, this code will over/re-iterate the data buffer.

Inspect Sources/Crypto/Util/PrettyBytes.swift at lines 45-51, starting with how the region iteration and Sequence traversal interact for non-contiguous DataProtocol values. Verify the iteration count and resulting hex string; done means each byte is converted once without changing the expected output.

Written by the indexing model from the issue text.