Make uses of Copy and Clone consistent in the public API

[conradoplg]

Currently, all structs that wrap Scalars implement Copy (and possibly a bunch of others). This can be confusing for users because you basically have to know if a struct implements Copy or Clone in order to use it.

For example, if you have a reference to a value v: &T, and want to pass it to a function with argument v: T, you need to pass *v if it's Copy but v.clone() if it's Clone.

There are multiple things we could to do address this:

• Leave as is
• Make every public struct Clone
• Make every public struct Copy
  • I don't think this is an option since some structs have maps which aren't Copy

I'm not sure what the best approach so we need to decide that first.

[conradoplg]

Removing Copy will be a pain:

• Error types would no longer be Copy (because they may have Identifiers) in them, and non-Copy error are a pain
• Zeroization also requires Copy types

Here all the Copy types:

• error types
• Identifier (Scalar)
• SigningShare (Scalar)
• VerifyingShare (Element)
• CoefficientCommitment (Element)
• Header
• Nonce (Scalar)
• NonceCommitment (Element)
• SigningCommitments (struct)
• GroupCommitmentShare (Element)
• SignatureShare (Scalar)
• Signature (struct)
• SigningKey (Scalar)
• VerifyingKey (Element)

We might want to remove Copy just from SigningCommitments and Signature which are structs are are the odd ones out. But I'm not sure if it's worth it.

In the end I'm inclined to not change anything.