Hashing the debug output for secrets can be done with `bitcoin_hashes`
not just `std`. Mention this in the obfuscated string output when
neither are available.

In array initialisation we use magic number 64, this is the secret bytes
length multiplied by 2.

Please note; we still use the magic number 32, left as such because it
is used in various ways and its not immediately clear that using a
single const would be any more descriptive.

Use `SECRET_KEY_SIZE * 2` instead of magic number 64.

The identifier `i` is predominantly used for indexing an array but we
are using it as a place holder for the iterated value of an array that
is then printed. The identifier `byte` is more descriptive.

Done in preparation for adding similar code to the `ecdh` module.

The `serialize_secret` method is a getter method, it does not do any
serialisation. However we use the method on secret keys and key types so
in order for the name to be uniform use the descriptive name
`secret_bytes`.

Rename `serialize_secret` to be `secret_bytes`.

Improve rustdocs on `display_secret` by doing:

- Minor improvements to the rustdocs to aid readability in the editor.
- Do not guarantee (`assert_eq!`) debug output

Currently printing the `SharedSecret` using `Display` or `Debug` prints
the real secret, this is sub-optimal. We have a solution for other
secrets in the project where printing is obfuscated and we provide a
`display_secret` method for explicitly printing.

Mirror the logic for other secrets and obfuscate the `SharedSecret` when printing.