Completes the `Executed` enum in the router. Adds an `Escape` struct. Both are
needed for testing purposes.
Documents the gas constants in intent and reasoning.
Adds modernized tests around key rotation and the escape hatch.
Also updates the rest of the codebase which had accumulated errors.
contracts was smashed out of ethereum-serai. Both have now been smashed into
individual crates.
Creates a TODO directory with left-over test code yet to be moved.
