Move `v` in `chainID` field

[pscott]

Currently we have the function get_chain_id() which fetches the chainId either from v or from chainID depending on the transaction type.
Ideally we'd like to move v to chainID, set chainID.length to vLength. I don't know if they share the same endianness.

This would simplify get_chain_id(), and allow us to call get_chain_id() in the plugins (this function would need to be exported in the ethereum-plugin-sdk).