code: Support for non-streaming design in `example-channel` app

[adizere]

Problem statement

Most newcomers to the codebase play with the channel-based example app. One part that we repeatedly found to be confusing the need to stream values.

https://github.com/informalsystems/malachite/blob/680087ba6471b8ae232d270eaea88e696fa0314c/code/examples/channel/src/app.rs#L108-L114

Part of the reason we chose to implement streaming-based approach is b/c this is what will be needed in the Starknet sequencer use-case, and it also covers some more advanced features of the library.

However, the streaming-based approach seems to be too advanced to be used as a newcomer example application.

Definition of done

• Simplify the channel-based app to work through a simple sending of the whole value
• Adapt the tutorial to remove any streaming assumption
• Adapt the tutorial to document the fact that the channel-based app could be modified to support streaming (i.e., as an advanced feature, but not implemented by default)

Prerequisites & background

Familiarity with the channel-based app is necessary for working on this issue, as well as with the contributions guide and the basic system architecture.

ADR 003 might be a useful prerequisite to working on this issue.

[romac]

Maybe we can tackle both this and https://github.com/informalsystems/malachite/issues/953 by switching the example app over to proposal-only mode?