Refactor the relayer's code

[vmarkushin]

The current version of ethereum-hyperspace's code is a mess, so it requires at least some high-level refactor:

Replace non-obviously "safe" panics and unwraps with proper error handling
Split big functions into small ones
Generalize repeating code
Add doc-comments on important functions
Move the items (structs, traits, etc.) to their corresponding modules
Optimize imports and remove dead code