Create a common base contract for portals and relays

[guidanoli]

📚 Context

As previously discussed in #33, portals and relays have isomorphic interfaces. We decided to name this common interface "Input Relay". However, currently portals and relays have different base interfaces.

✔️ Solution

We should remove the "Portal" and "Relay" interfaces, and add an "Input Relay" interface. The same goes for the base implementations.

📈 Subtasks

• [ ] Add IInputRelay.sol and InputRelay.sol
• [ ] Make portals and relays use IInputRelay.sol and InputRelay.sol
• [ ] Remove IPortal.sol and Portal.sol
• [ ] Remove IRelay.sol and Relay.sol