Restructure the processor handling to use a variant object

[edo9300]

Rewrite the whole process function of the processor to use instead tag dispatching on variant objects.
Each single processor message got rewritten as a struct, inheriting from a base templated Process struct, containing only the required data for that process, rather than using the generic structure that ended up needing to spam bitwise operations to pack as many arguments in a byte as possible. This makes things easier to follow and streamlines the addition of new processes:

• Define its struct with the parameters it expects by also providing a constructor
  • If the process requires an answer, inherit it from Process<true>, if it requires no answer, inherit it from Process<false>
• Add such struct to the processors variant type
• Declare and implement its corresponding bool process(Processors::NewProcess& arg); function