feat: implements process bls to execution change

[distributedstatemachine]

Took a stab at this and submitting a draft PR for feedback.

This PR closes #249

[ralexstokes]

nice work!

there are a few changes to go ahead and make now

after those, we will want to change the use of assert! to errors but we can handle that after the first round of updates

[ralexstokes]

nice updates! left some comments

[ralexstokes]

nice updates! we are getting closer to merge :)

I left some comments and also I think can tighten up our implementation to drop some use of clone and be a bit less verbose

1. keep signed_address_change as &mut SignedBlsToExecutionChange
   you can then immediately do this

let address_change = &mut signed_address_change.message;
let signature = &signed_address_change.signature;

// ... rest of code

2. let's follow the spec and grab a ref to our validator (it looks like this won't collide anywhere else as far as the borrow checker is concerned)

let mut validator = &mut state.validators[address_change.validator_index];

3. we can then also bind the withdrawal credentials so we don't have to keep going through the state data

let withdrawal_credentials = &validator.withdrawal_credentials;

4. let's not clone the from_bls_public_key,

let public_key = &address_change.from_bls_public_key;

you will need to clone it when assembling the InvalidBlsToExecutionChange error, but if you only clone inside the if body then we will only pay the cost when necessary

5. I may have misled you w/ the verify_signed_data helper as we will run into some issues w/ the borrow checker

instead lets just inline its implementation:

    let signing_root = compute_signing_root(address_change, domain)?;
    verify_signature(&address_change.from_bls_public_key, signing_root.as_ref(), signature).map_err(Into::into)

[yash-atreya]

Hey @ralexstokes have made the requested changes in PR #265

[distributedstatemachine]

@ralexstokes I have made changes , but want to clarify the following :

let's follow the spec and grab a ref to our validator (it looks like this won't collide anywhere else as far as the borrow checker is concerned)
let mut validator = &mut state.validators[address_change.validator_index];

IIUC, we already take a reference , and let mut validator generates a warning

warning: variable does not need to be mutable

verify_signature(&address_change.from_bls_public_key, signing_root.as_ref(), signature).map_err(Into::into)

I have mapped this to the actual error enum in state_transation::errors, as the suggestion generates this error

error: type annotations needed
label: cannot infer type of the type parameter `T` declared on the trait `Into`

[ralexstokes]

nice updates! left some nits below

[ralexstokes]

let's just clean up the errors and then this should be ready to merge!

[ralexstokes]

excellent! thanks for staying w/ me on all the back-and-forth

a few minor nits but I think can I merge from here