[decimal] Implement round() for BigDecimal

[forfudan]

This pull request includes several changes to the BigDecimal library, focusing on adding new functionalities, improving existing methods, and making the code more robust. The most important changes include the addition of new arithmetic operations, enhancements to rounding and comparison methods, and updates to the benchmarking script.

New Arithmetic Operations:

• Added truncate_divide and truncate_modulo functions to src/decimojo/bigdecimal/arithmetics.mojo for performing truncated division and modulo operations.
• Implemented __floordiv__ and truncate_divide methods in the BigDecimal struct to support floor division. [1] [2]

Enhancements to Rounding:

• Introduced a new round function in src/decimojo/bigdecimal/rounding.mojo to handle rounding with different modes and precisions.
• Added __round__ methods to the BigDecimal struct to support rounding with specified decimal places and rounding modes.

Improvements to Comparison Methods:

• Removed the raises keyword from several comparison functions in src/decimojo/bigdecimal/comparison.mojo to simplify their signatures. [1] [2] [3]

Updates to Benchmarking Script:

• Added a new benchmark for the round function in benches/bigdecimal/bench.mojo and updated the script to include the new benchmark. [1] [2] [3] [4]

Other Changes:

• Updated the BigDecimal struct to implement additional traits and improved the scale_up_by_power_of_10 function in src/decimojo/biguint/arithmetics.mojo to handle non-positive values. [1] [2] [3] [4] [5]

These changes enhance the functionality and robustness of the BigDecimal library, making it more versatile and easier to use.