Ivk to bytes visibility

[PaulLaux]

No description provided.

[what-the-diff]

• The Signed struct was made cloneable.
• A new method, from_parts, was added to the IssueBundle<T: IssueAuth> trait that allows for constructing an instance of this type from its constituent parts (i.e., ik, actions and authorization).
• A new method, to_bytes, was added to the IssuanceValidatingKey class which returns a byte array representation of itself by converting it into I2LEOSP format using its x-coordinate as input data (since y = 0 in our case). This is done because we need a way to convert an issuance validating key into bytes so that we can use it as part of the hash function used when creating commitments during note creation/issuance transactions on RedPallas blockchain network(s) - see https://github.com/redpallas1/zkvm-rs#commitment-generation for more details about how these are generated and why they're needed at all!

[PaulLaux]

Merged as part of #44