Solidity contract that accepts unverified substrate headers (#65)

* solidity contract

* continue

* upd

* cargo update

* fixes

* ehtereum_headers -> headers

* extracted some common stuff

* ethereum_sync.rs -> sync.rs

* make sync generic

* continue extracting

* continue

* add eth-contract argument

* continue

* some fixes

* contract v2

* continue

* more fixes

* more fixes

* deal with duplicated params

* removed multiple call_rpc variants

* bail_on_error!()

* fn submit_ethereum_transaction

* more fixes

* cargo fmt --all

* fix

* bail_on_arg_error!()

* fix

* fix

* remove async_extra stuff

* substrate-bridge.json -> substrate-bridge-abi.json

* get rid of substrate transactions hashes

* get rid of ethereum transactions hashes

* extracted contract bytecode to separate file

* cargo fmt --all

* avoid duplicate import in contracts

* removed Default::default()

* swapped configurations for sub2eth && eth2sub

* fix compilation

* do not double gas limit when submitting Substrate headers

* cargo fmt --all

* solidity contract removed

* consts

* extracted solc compilation details to separate file

* removed (obsolete in future Vec<u8> justification)

* fixed cli option description

* fix typos

* fix grumble

* extracted constants

* log decoded header

* cargo fmt --all

* comment