[integer] Implement several dunder methods for BigUInt

[forfudan]

This pull request introduces several enhancements and new features to the decimojo library, focusing on arithmetic operations, comparison functions, and benchmarking. The most important changes are the addition of the Toom-Cook 3-way multiplication algorithm, new power and comparison methods for the BigUInt structure, and updates to the benchmarking script.

Arithmetic Enhancements:

• src/decimojo/biguint/arithmetics.mojo: Added the multiply_toom_cook_3 function, implementing the Toom-Cook 3-way multiplication algorithm for efficient large number multiplication.

Power Methods:

• src/decimojo/biguint/biguint.mojo: Introduced __pow__ and power methods to the BigUInt structure, allowing exponentiation with both Int and BigUInt types. [1] [2]

Comparison Functions:

• src/decimojo/biguint/comparison.mojo: Added new comparison functions (greater, greater_equal, less, less_equal, equal, not_equal) to facilitate comparisons between BigUInt instances.

Benchmarking:

• benches/biguint/bench.mojo: Updated the benchmarking script to include the bench_multiply function for testing multiplication performance.

Documentation:

• README.md: Corrected a typo in the section heading "Nomenclature".