Separate private keys from the rest of the config

[norswap]

Currently, we have a bunch of configuration options for accounts and private key.

This is super redundant: the accounts can be derived from the private keys, and by default we should do this derivation.

We can still allow accounts to be specified as a safety option, in which case, we will verify that the account & key match.

To do this we should install & use the eth_account python package.

[norswap]

We could use this: https://www.rareskills.io/post/generate-ethereum-address-from-private-key-python

[norswap]

It honestly might be a good idea to write the accounts explicitly, but to check the private keys against the supplied accounts, to avoid errors. Also the private keys should probably be providable separately from the config file (probably via environment variables? a secrets.toml file?) so that they do not get mingled to the config file, which should be commitable.

I'm changing the title of this issue to reflect this.