[decimal] Implement factorial() and exp() function

[forfudan]

This pull request introduces the Decimal exponential function (exp) and factorial function (factorial), a comprehensive benchmarking suite for the Decimal exponential function (exp), and includes various improvements and optimizations across the codebase.

New Benchmark Script:

• Added a detailed benchmark script for the Decimal exponential function, comparing performance against Python's decimal module across 20 diverse test cases.

Project Configuration Updates:

• Added new test and benchmark entries for the exp and factorial functions.

Codebase Refactoring:

• Refactored the add function to handle special cases more efficiently and improve performance by avoiding unnecessary calculations.

Documentation and Licensing:

• Added a to-do list highlighting the need to optimize the multiply function for better performance in high-precision decimal multiplication.
• Updated the license information and added module-level documentation.

Additional Benchmark Case:

• Added a new benchmark case for decimal multiplication with many digits after the decimal point.