WIP: Stop bleeding the backends into user code

[RmbRT]

• Keep all backend template parameters entirely constrained to the internals of the SDK.
• Session and client constructors don't need backend template parameters
• Transaction receipts should be a uniform interface that only optionally exposes backend-specific types. It should have TX hash, block hash, block number maybe, sender, and the action that it corresponds to. All this information should be described using Erdstall types.
• Unify L1 and L2 TX handles and receipts etc. into one common abstraction. For example session.leave() is a mix of L2 TX (exit request), waiting for phase shifts, and then sending L1 TXs. Make this as easy as possible to interface with from a user and UI perspective. Maybe have a Process or something awaitable that models transactions and phase shift waits.
• Uniform and descriptive error types for L1 and L2 interactions.
• […]