[decimal] Implement ln for BigDecimal

[forfudan]

This pull request introduces several enhancements and fixes to the bigdecimal module, including the addition of a new natural logarithm function, renaming and refactoring of existing methods for clarity, and updates to dependencies and test cases.

New Features:

• Added a natural logarithm function to the bigdecimal module (ln). [1] [2]

Method Renaming and Refactoring:

• Renamed true_divide_fast to true_divide_inexact and updated its implementation to use number_of_significant_digits instead of minimum_precision. [1] [2] [3] [4] [5] [6] [7] [8]

Dependency Updates:

• Added mpmath as a new dependency in mojoproject.toml.

Error Message Improvements:

• Enhanced error messages in BigUInt struct methods for better clarity. [1] [2]

Test Case Updates:

• Updated test cases to include tests for the natural logarithm function.