General improvements to CI

[Amjad50]

• Updated codecov-action to v4
• Added clippy and fmt checks

[codecov]

Codecov Report

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

Project coverage is 81.81%. Comparing base (db5be91) to head (dbec9f3).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
plastic_core/src/misc.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #25      +/-   ##
==========================================
- Coverage   81.86%   81.81%   -0.05%     
==========================================
  Files          37       37              
  Lines        5349     5352       +3     
==========================================
  Hits         4379     4379              
- Misses        970      973       +3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.