Add `Span` functions where applicable

### New API Proposal: `Span` support

#### Motivation:

Lots of types, especially around keys, allow you to access the underlying storage with `func withUnsafeBytes(_ 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

Start by auditing Crypto types, especially key types, that expose withUnsafeBytes, and compare their APIs with Swift Span support. The issue does not name files or tests, so first identify the applicable entry points and existing API conventions. Done means safe Span-based alternatives are provided wherever appropriate and the relevant tests cover them.

Written by the indexing model from the issue text.