Swap out vector with hash set for `LocalVocab`

[RobinTF]

Based on the suggestion https://github.com/ad-freiburg/qlever/pull/1524#discussion_r1851855712 This should make merging vocabs more efficient.

[sparql-conformance[bot]]

Conformance check passed ✅

No test result changes.

Details: https://qlever.cs.uni-freiburg.de/sparql-conformance-ui?cur=f78f64d3211461fcab702d87ddcedfe443df550c&prev=d53d4f9fd91fd03463382e2b6f476d4ee345d6d9

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 89.25%. Comparing base (d53d4f9) to head (f78f64d).

Files with missing lines	Patch %	Lines
src/engine/LocalVocab.h	0.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1632 +/- ## ========================================== - Coverage 89.25% 89.25% -0.01% ========================================== Files 374 374 Lines 35659 35659 Branches 4027 4027 ========================================== - Hits 31829 31828 -1 - Misses 2529 2530 +1 Partials 1301 1301 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

---

🚨 Try these New Features:

• Flaky Tests Detection - Detect and resolve failed and flaky tests

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud