Test for 'Verifying a single transaction'; minor improvements

[pmikolajczyk41]

Description

When working on the exercises I encountered two annoying things:

• cargo was continually complaining about some small problems (even after completing exercise)
• 2nd exercise ('Verifying a single transaction') couldn't have been tested separately from the 3rd one

This PR:

• calms down cargo clippy as much as possible (i.e. after completing the exercises, there should be no warnings)
• introduces UnaryRollup which is a simplified version of Rollup - this one serves for testing 2nd exercise
• adds a unit test for the 2nd exercise using UnaryRollup
• fixes rendering of some READMEs

Checklist:

• [x] Targeted PR against correct branch (main)
• [x] Linked to Github issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• [ ] Wrote unit tests (n/a - typos and unit test itself)
• [x] Updated relevant documentation in the code
• [ ] Added a relevant changelog entry to the Pending section in CHANGELOG.md (n/a - seems to be unmaintained)
• [x] Re-reviewed Files changed in the Github PR explorer

[weikengchen]

Let me clean it up a bit, apparently the remaining tests have been failing for more than one year.

[pmikolajczyk41]

@weikengchen I'm afraid that unused variable is the thing that we should ignore in CI (or prepend corresponding object names with _), at least for such (tutorial) repository

I'm not so sure about no-std checks though

[weikengchen]

Let me merge it first. The no-std check is pretty weird, and I believe a larger refactoring is likely needed for this to be compatible with 0.4.0.

[pmikolajczyk41]

@weikengchen thank you!