Aries-VCX/Typestate Connection

[bobozaur]

Use Rust's typestate pattern for the Connection type.

Advantages:

• Compile time guarantees that the state machine cannot get into an invalid state.
• Better performance by greatly decreasing condition checking.
• Less and cleaner code (due to less nesting and more specialization)

Disadvantages:

• The state machine transitions from one type to another. Storing all of them in a container would require either conversion to/from a common type or other approaches such as Box<dyn Any> and downcasting.

Additional Goals:

Besides the advantages mentioned above, the linked PR will target a more thorough analysis and refactor of the overall Connection functionalities and aims to:

• deduplicate code
• handle memory better by a stricter use of clones (right now they're all over the place)
• comment structs and methods

Other ideas:

• Restrict a spawned connection to only a certain type of transport (HTTP, Websockets, SSH, etc.)? Should it be allowed for a Connection that's been used over some transport, e.g. HTTP, to switch to a different one? I'd say that should require a different Connection.