feat: Faster computation of L0

[davidnevadoc]

Solves #269

This optimizations yields some modest imrpovements in key geneartion. For K=20

	Keygen	L0 gen
Previous	32.38 s	1.82 s
This PR	30.81 s	774 ms

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

Attention: Patch coverage is 97.91667% with 1 line in your changes missing coverage. Please review.

Project coverage is 82.67%. Comparing base (bc857a7) to head (6df6663).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
halo2_backend/src/poly/domain.rs	97.77%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #363      +/-   ##
==========================================
+ Coverage   82.64%   82.67%   +0.02%     
==========================================
  Files          84       84              
  Lines       17750    17786      +36     
==========================================
+ Hits        14670    14705      +35     
- Misses       3080     3081       +1     

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

[guorong009]

LGTM! 🚀