Minimal API changes for Issuance in Client

[alexeykoren]

• Added a Copy trait to be able to work with IssuanceAuthorizingKey and SigningKey in a same manner as with others
• from_bytes made public for Client

[PaulLaux]

approved with comments.

[PaulLaux]

/// Constructs an Orchard issuance validating key from uniformly-random bytes.
///
/// Returns `None` if the bytes do not correspond to a valid key .

Is better adjusted to the existing style.

[PaulLaux]

let's go with

let issue_bundle = sign_issue_bundle(unauthorized, rng, keys.isk());
...
fn sign_issue_bundle(
    unauthorized: IssueBundle<Unauthorized>,
    mut rng: OsRng,
    isk: &IssuanceAuthorizingKey,
) -> IssueBundle<Signed> {
    let sighash = unauthorized.commitment().into();
    let proven = unauthorized.prepare(sighash);
    proven.sign(&mut rng, isk).unwrap()
}