Fix string pool lookup performance

[josephbirkner]

The naive lookup using a lower-case string copy is now really starting to impact performance during model building quite badly. So, here is a fix :smile:

[github-actions[bot]]

Test Results

 1 files  ±0   1 suites  ±0   3m 12s ⏱️ +13s 36 tests ±0  36 ✅ ±0  0 💤 ±0  0 ❌ ±0  37 runs  ±0  37 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit 9363b221. ± Comparison against base commit df138085.

:recycle: This comment has been updated with latest results.

[sonarcloud[bot]]

Quality Gate passed

Issues
12 New issues
43 Accepted issues

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

See analysis details on SonarCloud

[github-actions[bot]]

Package	Line Rate	Branch Rate	Health
include.simfil	64%	47%	➖
include.simfil.model	87%	61%	✔
src	72%	41%	➖
src.model	83%	51%	✔
Summary	74% (2141 / 2875)	44% (1776 / 4017)	➖

Minimum allowed line rate is 50%