[integer] Optimize BigUInt type and its arithmetic operations

[forfudan]

This pull request includes several changes to the DeciMojo library, focusing on improving the BigUInt type and its arithmetic operations. The most important changes involve refactoring methods for better performance and clarity, and updating documentation.

Documentation Updates:

• README.md: Updated the description of BigInt to specify that it is a base-10 arbitrary-precision integer type. Added a detailed explanation of the nomenclature and the design philosophy behind DeciMojo. [1] [2]

Code Refactoring:

• src/decimojo/biguint/arithmetics.mojo: Replaced manual trailing zero removal with a new remove_trailing_zeros method in several functions (subtract, multiply, floor_divide, general_divide). [1] [2] [3] [4]
• src/decimojo/biguint/arithmetics.mojo: Simplified the multiply function by directly returning BigUInt(UInt32(0)) for zero operands.
• src/decimojo/biguint/arithmetics.mojo: Refactored floor_divide to handle single-word divisor cases more efficiently and renamed general_divide to floor_divide_general. [1] [2] [3] [4] [5] [6]

New Methods:

• src/decimojo/biguint/arithmetics.mojo: Added floor_divide_inplace_by_2 to perform in-place division by 2 for BigUInt.

Constructor and Initialization Changes:

• src/decimojo/biguint/biguint.mojo: Updated BigUInt constructors to differentiate between raw and validated word initialization. Introduced from_words for validated initialization and updated the documentation accordingly. [1] [2] [3] [4] [5]

Additional Enhancements:

• src/decimojo/biguint/biguint.mojo: Added new utility methods __rshift__ for right shift operation and is_two to check if the value is two. [1] [2]
• benches/biguint/bench.mojo: Included bench_biguint_add in the benchmark suite.