pset: blind: return ephemeral private key

[LeoComandini]

Right now we have:

• Transaction::blind returns abf, vbf and ephemeral private key
• PartiallySignedTransaction::blind_non_last returns abf and vbf
• PartiallySignedTransaction::blind_last returns nothing

This PR makes things more consistent and makes ephemeral keys more easily accessible.

It's a breaking change, but it's just augmenting the data, so it should be easy to handle downstream.

[apoelstra]

In a followup (or another commit, in case you get to this before I merge it), we should refactor TxInType to just be a flat enum, and have a separate struct with the index.

I think maybe

pub enum CtLocationType {
    Input,
    Issuance,
    Reissuance, // note I changed capitalization here
}

pub struct CtLocation {
    input_index: usize,
    ty: CtLocationType,
}

I'm not married to these names, but I think they're clearer than TxInType.

[LeoComandini]

Refactored TxInType to CtLocation

I've no strong opinion on the name, rn I can't think of something better than CtLocation so I used that

[apoelstra]

ACK e4144e6; successfully ran local tests

[apoelstra]

ACK cc12b8e; successfully ran local tests