Change the u256 implementation, and prepare new beta release

[tlepoint]

• Use https://crates.io/crates/ethnum for u256, this accelerates the scaling operation, and gives speedup by 5+% for the homomorphic multiplication
• Prepare the new beta release (since currently the proto dependencie that was not pinned make the crate fail)

[codecov]

Codecov Report

Merging #207 (103ce9a) into main (92d3b61) will decrease coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #207      +/-   ##
==========================================
- Coverage   94.30%   94.25%   -0.06%     
==========================================
  Files          34       33       -1     
  Lines        7695     7588     -107     
==========================================
- Hits         7257     7152     -105     
+ Misses        438      436       -2     

Files	Coverage Δ
crates/fhe-math/src/rns/scaler.rs	98.58% <100.00%> (-0.04%)	⬇️
crates/fhe-util/src/lib.rs	100.00% <ø> (ø)