[integer] Implement floor_divide_by_power_of_billion()

[forfudan]

This pull request introduces several updates to the decimojo library, focusing on improving numerical operations with BigUInt and BigDecimal types. Key changes include replacing a placeholder function with a more optimized implementation, adding debug assertions to enforce non-negative input constraints, and introducing a new utility function for division by powers of a billion.

Improvements to numerical operations:

• Optimized division by powers of a billion: Replaced the placeholder call to floor_divide_by_power_of_ten with the newly implemented floor_divide_by_power_of_billion in true_divide_fast for better performance.

• New utility function: Added floor_divide_by_power_of_billion, which efficiently removes the last n words of a BigUInt, equivalent to dividing by (10^9)^n. This function handles edge cases and includes debug assertions.

Input validation and robustness:

• Debug assertions for non-negative inputs: Added debug assertions to ensure that input parameters n are non-negative in functions such as multiply_by_power_of_ten, multiply_inplace_by_power_of_ten, multiply_by_power_of_billion, multiply_inplace_by_power_of_billion, and floor_divide_by_power_of_ten. These assertions improve code reliability during development and testing.

• Updated handling of zero or negative n: Modified behavior in several functions to return the input unchanged when n <= 0, ensuring consistent and predictable results.

These changes collectively enhance the library's performance, robustness, and maintainability.