[decimal] Implement exp() for BigDecimal

[forfudan]

This pull request includes several updates to the DeciMojo project, focusing on enhancing the documentation, adding new functionality, and improving performance. The most important changes are summarized below:

Documentation Enhancements:

• README.md: Updated table of contents and added <!-- omit in toc --> comments to headings to improve navigation and readability. [1] [2] [3] [4]

New Functionality:

• src/decimojo/bigdecimal/bigdecimal.mojo: Added exp method to the BigDecimal struct to calculate the exponential of a BigDecimal number.
• src/decimojo/bigdecimal/rounding.mojo: Implemented round_to_precision function for rounding BigDecimal numbers to a specified precision.
• benches/bigdecimal/bench.mojo: Added benchmarks for the new exponential function. [1] [2] [3]

Performance Improvements:

• src/decimojo/biguint/arithmetics.mojo: Optimized add and add_inplace functions for BigUInt by adding short-circuit cases and an optimized method for adding 1. [1] [2] [3]

Miscellaneous:

• docs/todo.md: Added new to-do items for future enhancements, including implementing a global precision variable for BigDecimal and adding a remove_trailing_zeros method for BigUInt.
• src/decimojo/bigdecimal/exponential.mojo: Added detailed implementation for the exp function and supporting Taylor series calculation. [1] [2] [3]

These changes collectively improve the functionality, performance, and documentation of the DeciMojo library.