[integer] Ensure that BigUInt has no leading zero words

[forfudan]

This pull request focuses on improving the BigUInt and BigDecimal implementations in the decimojo library by enhancing precision, optimizing arithmetic operations, and ensuring consistency in handling zero values. It also includes a minor documentation update. Below is a categorized summary of the most important changes:

Zero words:

• Update logic to ensure that there are no leading zero words in BigUInt by design. Add debug assert to check this.
• Update is_zero() and comparison functions given that there are no leading zero words in BigUInt by design.

Bug Fix:

• Fixed a syntax error in the extend_precision example in BigDecimal by correcting a missing closing quote.

Consistency Improvements:

• Replaced instances of BigUInt() with BigUInt.ZERO for clearer intent when representing zero values across multiple functions.

Code Simplification and Optimization:

• Replaced intermediate List[UInt32] allocations with direct BigUInt instantiations in arithmetic functions like add_slices_simd, subtract_school, and multiply_slices_school. This reduces unnecessary memory usage and improves performance.
• Updated arithmetic methods to remove leading zero words after operations to ensure normalized results. This applies to addition, subtraction, and multiplication operations.

API Enhancements:

• Introduced BigUInt.from_slice for copying slices of words, replacing manual slice handling in add_inplace_by_slice. This improves readability and reduces potential errors.

Documentation Update:

• Added a link to the project's Discord channel in README.md for community engagement.

These changes collectively enhance the library's efficiency, maintainability, and usability.