Change Prevouts::All(&[TxOut]) to Prevouts::All(&[&TxOut])

[sanket1729]

I believe this avoids some allocation of creating a vec of TxOut to
create a slice incase the data is already available in psbt/other
methods.

See #834

[tcharding]

This PR closes #834, right? You used 'see #834', so I was left wondering?

[Kixunil]

I'm not convinced by this. This change helps the caller who already has &[&TxOut] while before it benefited the caller already having &[TxOut]. Either case requires conversion to the other.
From quick look I believe T: Iterator, T::Item: Borrow<TxOut> should work. I suggest to use that unless there's something I don't see.

[sanket1729]

This change helps the caller who already has &[&TxOut] while before it benefited the caller already having &[TxOut].

Yup, exactly I am not sure how to evaluate this. For the psbt workflow, Having [& [&TxOut]] is more useful because the txouts are in separate fields of the pbst data structure. Converting from &[TxOut] to &[&TxOut] seems much better than going the other way.

I can also imagine someone asking a full node gettxout and they have &[TxOut]. I see that this is not strictly better, which is why I raised #834 for more discussion.

[Kixunil]

IMO we should try generics with Iterator first and see if it can work.

[sanket1729]

@Kixunil, iterator turned out more annoying(requiring exact size iterators and cloning while consuming) and unneeded for this application. But the borrow::Borrow helped a lot. I believe this satisfies my use-case while retaining the current use-case too.

[Kixunil]

Oh, yeah, Borrow on item in the array looks good!

[sanket1729]

This clone was done because the generic T needed to be the same type across if else arms

[Kixunil]

Doesn't matter much in test anyway.

[sanket1729]

Yeah, just highlighted because it might confuse review.

[apoelstra]

This is kinda unfortunate because it might be a common pattern, but I don't see any sensible way around it.

[sanket1729]

The taproot_signature_hash function also gets the correct data for sighash single from prevouts::All. I think it is required to extract the corresponding txout from an array in this fashion. This was only done to test that both of these work.

[apoelstra]

ACK 10fedfb

TIL about Borrow. Seems like it's a strictly more useful AsRef.

[Kixunil]

ACK 10fedfb