chore(ci): limit the max number of features tested cargo check (#367)

With each added feature flag, the number of checks that needs to be
perfomed increased exponentially. This causes the "Build with
each feature combination" job to take up 1.5 hours!

This commit sets the depth (max number of features tested at once)
to 3, decreasing the total number of checks from 794 to 232, which
should greatly reduce the pipeline times without reducing the check
usefulness too much.

We might be forced to reduce this even more (to two) in the future if we
keep increasing the number of feature flags. We might think about
splitting the code into more crates then.

Author: m4tx

.github/workflows/rust.yml

@@ -320,7 +320,7 @@ jobs:
         uses: mozilla-actions/sccache-action@v0.0.9
 
       - name: Run cargo check with every combination of features
-        run: cargo hack check --feature-powerset --exclude-features db --no-dev-deps
+        run: cargo hack check --feature-powerset --depth 3 --exclude-features db --no-dev-deps
 
   miri:
     if: github.event_name == 'push' || github.event_name == 'schedule' || github.event.pull_request.head.repo.full_name != github.repository