Upgrade `libsecp256k1` from 0.3.5 ➔ 0.7.0 (#967)

* Upgrade `libsecp256k1` from `0.3.5` ➔ `0.7.0`

* Include off-chain dependencies only when necessary

* Include off-chain dependency only when necessary

* Include off-chain dependency only when necessary

* Switch to `secp256k1` for `eth_compatibility` crate

* Remove conditional dependency

* Fix serialize

* Switch to `secp256k1` crate everywhere

* Switch to `secp256k1` crate everywhere

* Fix compilation for `wasm32` target

* Ensure enabled features are not merged for targets

* Make clippy happy

* Use resolver `2` in all examples

* Switch to `libsecp256k1` 0.7.0

* Use default features

* Bring comment back

* Reduce diff

* Remove unnecessary import

* Update rustfmt config to edition 2018

* Use resolver `2` for workspace