[WIP] enable generalized explanations for linear propagator

[AllenZzw]

• update transform_lit to return the strongest possible LitMeaning that is equivalent to the input.
• add relaxed_lit to return the weakest LitMeaning that is equivalent to the input.
• add tests for transform_lit, relaxed_lit and rev_transform_lit functions
• enable generalized explanation for the IntLinearLessEqBoundsImpl propagator

[codecov]

Codecov Report

Attention: Patch coverage is 97.81659% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/huub/src/helpers/linear_transform.rs	98.96%	2 Missing ⚠️
crates/huub/src/propagator/int_lin_le.rs	92.59%	2 Missing ⚠️
crates/huub/src/model/int.rs	50.00%	1 Missing ⚠️

Files with missing lines	Coverage Δ
crates/huub/src/solver/engine.rs	83.36% <100.00%> (+2.10%)	⬆️
crates/huub/src/model/int.rs	53.13% <50.00%> (-1.69%)	⬇️
crates/huub/src/helpers/linear_transform.rs	97.25% <98.96%> (+10.76%)	⬆️
crates/huub/src/propagator/int_lin_le.rs	95.09% <92.59%> (-0.56%)	⬇️

... and 6 files with indirect coverage changes

[codspeed-hq]

CodSpeed Performance Report

Merging #118 will improve performances by ×18

_{Comparing AllenZzw:feat/generalized_int_lin_le_explanations (9f53984) with develop (97bfd35)}

Summary

⚡ 2 improvements
✅ 11 untouched benchmarks

Benchmarks breakdown

	Benchmark	develop	AllenZzw:feat/generalized_int_lin_le_explanations	Change
⚡	optimization[svrp_s4_v2_c3]	595.5 ms	88 ms	×6.8
⚡	satisfaction[sudoku_p48]	179.3 ms	10.2 ms	×18

[AllenZzw]

There are two functions which we may need to clarify:

• There are multiple literals that are valid returns for the transform_lit function.
• The get_int_lit_relaxed function may return constant true literal in two cases: (1) when the input refers to a newly created lazy literal which has not been assigned true yet, and (2) when the input bounds are out of the original integer domains. The first case may relate to a more general problem on whether the get_* interfaces have consistent returns about the state.