New Introduction Protocol
These are so far only the first commits for a new Introduction Protocol. They only add message/session encoding/parsing.
I am starting this in a new package and will only remove the old one in the end, so we could merge this consecutively. Anyway, opening an MR already to get early feedback and having the option to ask questions about specifics.
Closes #308 (closed), #377 (closed), #474 (closed), #613 (closed)
Closes? #376 (closed)