Implement u256 prime field

[GianfrancoBazzani]

Implement u256 prime field

Description

solves #126
solves #95

Implemented U256PrimeField and generalized Montgomery backed for avoiding code repetition and allow easy and fast future implementations of other fields with different limb lengths.

Example of the UX while using the mod:

trait aliases are experimental see issue #41517 rust-lang/rust#41517, but in the future the use of trait alias may upgrade the UX to something like this, note that instead of use general trait IsMontgomeryConfiguration<"limbs"> a trait alias would be defined, for example U256IsMontgomeryConfiguration = IsMontgomeryConfiguration<4>:

Type of change

• New feature

Checklist

• Linked to Github Issue
• Unit tests added
• This change requires new documentation.

[codecov-commenter]

Codecov Report

Merging #137 (70984d3) into main (3c681a3) will decrease coverage by 0.70%.
The diff coverage is 91.25%.

@@            Coverage Diff             @@
##             main     #137      +/-   ##
==========================================
- Coverage   95.71%   95.02%   -0.70%     
==========================================
  Files          49       49              
  Lines        5251     5382     +131     
==========================================
+ Hits         5026     5114      +88     
- Misses        225      268      +43     

Impacted Files	Coverage Δ
crypto/src/hash/poseidon/mod.rs	96.81% <ø> (ø)
...rt_weierstrass/curves/bls12_377/field_extension.rs	75.00% <ø> (ø)
...rt_weierstrass/curves/bls12_381/field_extension.rs	10.00% <ø> (ø)
...tic_curve/short_weierstrass/curves/test_curve_2.rs	88.00% <ø> (ø)
proving-system/stark/src/lib.rs	96.99% <ø> (ø)
...src/field/fields/montgomery_backed_prime_fields.rs	91.25% <91.25%> (ø)
math/src/field/element.rs	87.71% <0.00%> (-4.39%)	⬇️

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[MauroToscano]

Looks good!

As a side note, use git move when renaming files, so git realizes it's the same one edited.