New function: script.solveOutput

[afk11]

Replaces #697 because that targeted another base.

[afk11]

This tests both sighash implementations against live vectors, plus the test logic may well end up incorporated into signing somewhere

[afk11]

Rebased

[afk11]

Rebased, time for merge?

[afk11]

One of the motivations of this PR is to make stuff like this easier:
https://github.com/bitcoinjs/bitcoinjs-lib/blob/master/src/transaction_builder.js#L150

We could run solveOutput, save that as prevOutput, then we get the following without having to duplicate work:

 {
    type: outputType,
    script: scriptCode,
    solvedBy: solvedBy,
    requiredSigs: requiredSigs
}

[dcousens]

@afk11 we could maybe make it private for now, try out the use cases then decide?

[afk11]

I still think we need this, mind merging it?

[dcousens]

ACK, but I don't think the name is right as it implies solveOutput(ouput) -> solvedOutput ... aka, it is a verb.
It is actually akin whatSolvesThisOutput(output).

Also, #868 for use case and requested change (add redeemScript param, and maybe witnessScript too?)

[dcousens]

@afk11 thoughts on this?
It is useful, but, I struggled with the exact use-case... maybe an integration test?