Transaction signing is something that happens in a lot of places - this
PR introduces primitives for transaction signing in `transaction_utils`
such that we can use the same logic across web3/eth1/etc for this simple
operation.
`transaction_utils` also contains a few more "spec-derived" helpers for
working with transactions, such as the computation of a contract address
etc that cannot easily be introduced in `transactions` itself without
bringing in dependencies like secp and rlp, so they end up in a separate
module.
Finally, since these modules collect "versions" of these transaction
types across different eips, some tests are moved to follow the same
structure.
Jacek Sieka