Add arbitrary impl for types used in PSBTs

[shinghim]

No description provided.

[apoelstra]

concept NACK these ones -- we aren't allowed to put arbitrary data into these structures. All the C library guarantees us is that the size is OK to assume. In particular, this will produce invalid x-only public keys which will cause aborts if used with the rest of the library.

[shinghim]

Oh i see. Would it be acceptable to create random SecretKey from the doc example:

/// let secp = Secp256k1::new();
/// let secret_key = SecretKey::new(&mut rand::rng());

and use that to generate the arbitrary types in secp256k1 and leave the ones in ffi alone?

[apoelstra]

@shinghim yeah, secret keys in particular I think you can go ahead and produce using Arbitrary. The upstream C library just uses unsigned char* for these.

For the other FFI types, I think you can do it, but you need to use a similar strategy to the main crate, where you generate byte arrays then use the (unsafe) decoding functions to produce the actual objects.

[shinghim]

Updated to derive XOnlyPublicKey from an arbitrary PublicKey and added an impl for SecretKey. I removed the impl for Signature since I didn't think there would be any use for an arbitrary one. I think it might more make sense to generate an arbitrary SecretKey and use that to create an "arbitrary" Signature

[apoelstra]

In 40f073b:

Sorry to keep iterating on this, but could you:

• Derive XOnlyPublicKey directly by attempting to parse a [u8; 32]
• Derive PublicKey by deriving a Parity and XOnlyPublicKey and using those to construct a PublicKey.

Currently you have the opposite -- to get a XOnlyPublicKey you construct a PublicKey, which consumes a byte to determine parity, which you then just forget.

[shinghim]

No worries, I appreciate the feedback and the review. Updated to create the PublicKey with an XOnlyPublicKey and Parity