Clippy fixes

[greg-szabo]

Closes #406 and preferably #193 . (Todo: decide if this closes #193 based on discussion there.)

What does this do?

• It keeps using clippy-check while addressing the issues mentioned in Fail on Clippy errors in CI #406 and Explore clippy rules and apply quality of life recommendations #193.

• It turns the warnings in clippy-check into errors.

• It clarifies what to look for:

  • The name of the task is now clippy-json-output to indicate that this is not the execution you want to check. (This is the task that executes clippy and produces a JSON output for parsing.)
  • The task creates a new clippy-results GitHub Action that renders the JSON output into a nice web view. (This is what should be checked and referred to.)

• Now that all warnings are errors, I had to fix clippy warnings (turned errors) described here and here

• For some reason the clippy-results task got created in the "Test" section in the latest run while it was created in the "Rust" section before (where the original trigger task is). This might be some internal GitHub Actions stuff, I consider it a harmless side-effect.

• Referenced an issue explaining the need for the change

• Updated all relevant documentation in docs

• Updated all code comments where relevant

• Wrote tests

• Updated CHANGELOG.md

[codecov-commenter]

Codecov Report

Merging #868 (aca24b2) into master (4fa065b) will decrease coverage by 0.0%.
The diff coverage is 100.0%.

❗ Current head aca24b2 differs from pull request most recent head 7929932. Consider uploading reports for the commit 7929932 to get more accurate results

@@           Coverage Diff            @@
##           master    #868     +/-   ##
========================================
- Coverage    67.0%   67.0%   -0.1%     
========================================
  Files         212     212             
  Lines       16771   16771             
========================================
- Hits        11244   11243      -1     
- Misses       5527    5528      +1     

Impacted Files	Coverage Δ
proto/src/serializers/part_set_header_total.rs	100.0% <ø> (ø)
light-client-js/src/lib.rs	6.6% <100.0%> (ø)
rpc/src/endpoint/blockchain.rs	40.0% <0.0%> (-60.0%)	⬇️
rpc/src/endpoint/validators.rs	20.0% <0.0%> (-30.0%)	⬇️
testgen/src/header.rs	78.1% <0.0%> (-0.7%)	⬇️
light-client/src/types.rs	30.1% <0.0%> (-0.6%)	⬇️
tendermint/src/block/commit_sig.rs	79.5% <0.0%> (+2.2%)	⬆️
rpc/src/endpoint/abci_query.rs	21.4% <0.0%> (+3.7%)	⬆️
rpc/src/endpoint/broadcast/tx_async.rs	100.0% <0.0%> (+60.0%)	⬆️

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[thanethomson]

LGTM! 👍