Make parsing generic on accounts

[fedgiac]

Related to this comment and discussed here.

This PR makes parsing generic in the account vector. This helps back-end use the parser when AccountView isn't available.

Motivation

Right now, instruction parsing is done by parsing accounts and data in parallel. Parsing depends on both, in principle there isn't a way to "just parse the data, not the accounts," at least not now. For example, the way the data is parsed could depend on the number of accounts.

The main motivation for Ilya's request is that there's no meaningful way to get an AccountView off-chain, so even if parsing is part of the interface, it can't be relied upon for parsing instruction data. Here we make parsing generic, no more AccountView but some generic A. This works nicely for the program, because the parsing doesn't care about the account type, it just reorganizes the accounts into their specific role. So the back-end should be able to just give it the instruction's account list or whatever type it uses, which the back-end team confirmed to be available at the time they needs to parse the data.

How to test

CI. A passing review from someone from the back-end team.

[squadgazzz]

Thanks! LGTM, but let me quickly try to adapt it from the indexer side to ensure we don't miss anything else.

[squadgazzz]

Tested, should be g2g. cowprotocol/services#4666
Thanks again!

[kaze-cow]

amazing to me how this seemingly little change makes such a big difference, but I suppose it makes sense when you consider that the accounts aren't being manipulated at all. We are just rearranging them for parsing purposes. Hopefully this doesn't become an issue later for some unforseen future change would be my only reservation here 😆