* Unify error enums in substrate and ethereum clients with `thiserror`

Related to https://github.com/paritytech/parity-bridges-common/issues/857

* Add license pre-amble

* rustfmt

* Fix spelling

* shared conversion rate metric value

* clippy

greedy relayer don't need message dispatch to be prepaid if dispatch is supposed to be paid at the target chain (#1016)

* introduce relayer mode enum

* removed MaximalReward mode

* TODO

* Saturating -> SaturatingAdd

* ref issue in TODOs

* Update relays/messages/src/message_race_delivery.rs

Co-authored-by: Hernando Castano <[email protected]>

Co-authored-by: Hernando Castano <[email protected]>

* Remove chains.

* Move relay clients.

* Flatten generic.

* Fix fmt.

* use runtime:: prefix for message-lane pallet traces

* renamed message-lane (module and primitives) folder into messages

* replace "message lane" with "messages" where appropriate

* Nest some crates.

* Alter command execution to make it easier to add new bridges.

* Rename sub-dirs.

* cargo fmt --all

* Address clippy.

* Update relays/substrate/src/rialto_millau/cli.rs

Co-authored-by: Hernando Castano <[email protected]>

Co-authored-by: Hernando Castano <[email protected]>

* limit messages in the batch by weight/count

* fixed components compilation

* reverted obsolete parts of #469

* implement generated_messages_weights

* actually use computed weight in message proof

* fmt and clippy

* fixed TODO

* clippy

* Update relays/messages-relay/src/message_race_loop.rs

Co-authored-by: Hernando Castano <[email protected]>

* add issue reference

* add assert message

* grumbles

* fmt

* reexport weight from bp-message-lane

Co-authored-by: Hernando Castano <[email protected]>
Co-authored-by: Hernando Castano <[email protected]>

* message lane metrics

* fmt and clippy

* extracted relay crates

* moved metrics to utils

* exchange-relay compilation

* fix compilation of headers-relay

* fixed messages-relay compilation

* fixed ethereum-poa-relay compilation

* cargo lock

* cargo fmt --all

* clippy

* cargo fmt --all

* fix tests compilation

* clippy

* eof

* module level docs

* removed obsolete comment

* #![warn(missing_docs)]

* .0 -> Deref

* post-merge fix

* cargo fmt

* Update relays/headers-relay/src/headers.rs

Co-authored-by: Hernando Castano <[email protected]>

* Update relays/headers-relay/src/headers.rs

Co-authored-by: Hernando Castano <[email protected]>

* Update relays/headers-relay/src/lib.rs

Co-authored-by: Hernando Castano <[email protected]>

Co-authored-by: Hernando Castano <[email protected]>