Add `Span` functions where applicable

[0xTim]

New API Proposal: Span support

Motivation:

Lots of types, especially around keys, allow you to access the underlying storage with func withUnsafeBytes<R>(_ body: (UnsafeRawBufferPointer) throws -> R) rethrows -> R. Span was introduced to support these exact scenarios in a safe way and Crypto should support it

Importance:

Unsafe code is unsafe, safe alternatives should be provided

[Lukasa]

Yep, this is a reasonable feature request. Let me figure out what the right way is to handle it.