[integer] Refine the constructors of BigUInt

[forfudan]

This pull request includes several changes to improve the functionality and performance of the BigUInt and BigInt benchmarks and arithmetic operations. The most important changes include adding a new benchmark for the BigUInt from_string constructor, modifying arithmetic functions to use a more efficient approach for handling word lists, and making adjustments to the BigInt initialization.

New Benchmark Addition:

• benches/biguint/bench_biguint_from_string.mojo: Added a comprehensive benchmark for the BigUInt from_string constructor, comparing its performance against Python's built-in int() constructor with 20 diverse test cases.

Benchmark Command Updates:

• benches/bigint/bench.mojo and benches/biguint/bench.mojo: Updated the input prompt to ask for the name of the benchmark instead of the number, and added the new fromstr benchmark option. [1] [2]

Arithmetic Function Improvements:

• src/decimojo/biguint/arithmetics.mojo: Modified add, subtract, multiply, floor_divide, and divmod functions to use List[UInt32] for word lists instead of initializing empty BigUInt objects, improving performance and memory usage. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

BigInt Initialization Adjustments:

• src/decimojo/bigint/bigint.mojo: Adjusted the BigInt initialization to directly assign the magnitude using a list of words and updated the from_uint128 method to use BigUInt.from_scalar. [1] [2] [3]