[decimal] Re-implement the true_divide() function for BigDecimal

[forfudan]

This pull request introduces enhancements to the BigDecimal division functionality, including refactoring of the true_divide function and the implementation of two new division methods: true_divide_fast and true_divide_general. These changes aim to improve precision, performance, and code maintainability.

Division Function Refactoring:

• Refactored the true_divide function to simplify logic and delegate division tasks to specialized helper functions (true_divide_fast and true_divide_general). This improves readability and separates concerns.

New Division Methods:

• true_divide_fast: Introduced a high-performance division method that avoids rounding and trailing zero removal, suitable for cases where speed is prioritized over formatting.
• true_divide_general: Added a precise division method that rounds results to the specified precision and removes unnecessary trailing zeros, ensuring compliance with exact division requirements.

Code Simplification:

• Replaced unused imports (time and testing) with math in arithmetics.mojo, aligning dependencies with the updated implementation.

Benchmark Enhancements:

• Added new benchmark cases (Cases 57–64) to test division of BigDecimal numbers with varying word sizes, including extremely large numbers. These cases use the newly introduced ITERATIONS_LARGE_NUMBERS constant for iterations.