Run clippy in continuous integration workflow

[Anders429]

Fixes some clippy lints and enables clippy in CI. This is the last step of #9.

[Anders429]

Looks like this is failing because clippy is being run on stable, while I tested locally on nightly. This is because associated types were recently ignored in this lint (see this PR), but that change doesn't seem to have bubbled up into stable yet. I'll go ahead and switch the workflow to run on nightly instead.

[codecov-commenter]

Codecov Report

Merging #44 (a80d1e6) into master (848dc68) will decrease coverage by 0.17%.
The diff coverage is 10.52%.

@@            Coverage Diff             @@
##           master      #44      +/-   ##
==========================================
- Coverage   19.28%   19.10%   -0.18%     
==========================================
  Files          50       50              
  Lines        3672     3664       -8     
==========================================
- Hits          708      700       -8     
  Misses       2964     2964              

Impacted Files	Coverage Δ
src/archetype/identifier/iter.rs	100.00% <ø> (ø)
src/archetype/identifier/mod.rs	91.00% <ø> (ø)
src/archetype/mod.rs	14.74% <ø> (ø)
src/entity/allocator/mod.rs	0.00% <0.00%> (ø)
src/registry/debug.rs	0.00% <0.00%> (ø)
src/registry/eq.rs	0.00% <0.00%> (ø)
src/registry/serde.rs	0.00% <0.00%> (ø)
src/system/schedule/sendable.rs	0.00% <0.00%> (ø)
src/system/schedule/stage/seal.rs	0.00% <0.00%> (ø)
src/system/schedule/task.rs	0.00% <0.00%> (ø)
... and 2 more

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[Anders429]

Two of the three failures are related to a regression on the latest nightly builds of clippy. See this issue. I believe the third is also a false-positive, as Option<T> implements Copy if T implements Copy, and Location is Copy. Therefore, removing the reference is actually not the same thing logically. Here is a related issue.

Perhaps I should stick to stable instead? Or at least the beta channel? It seems that nightly may be a little more unstable than I hoped for.

Edit: looks like beta has its own can of worms. Perhaps clippy may have to wait? Not sure what else to do here.

[Anders429]

I'm going to keep this workflow running on nightly. It looks like these lints will be fixed and live on nightly soon. This merge can wait until then.

[Anders429]

Rather than waiting, I will put an allow lint for these, with a todo comment referencing the relevant issues. That will unblock this.