[decimal] Implement some IO methods for BigDecimal

[forfudan]

This pull request introduces significant changes to the decimojo library, primarily focusing on the addition of the BigDecimal type and various improvements to the BigInt and BigUInt types. The changes include new functionality, refactoring, and code cleanup.

New Features:

• BigDecimal Implementation:
  • Added a new BigDecimal type with comprehensive methods for construction, arithmetic operations, and type conversion. (src/decimojo/bigdecimal/bigdecimal.mojo)

Refactoring and Code Cleanup:

• BigInt Enhancements:

  • Refactored BigInt to handle the case of Int.MIN more robustly by introducing an is_min flag. (src/decimojo/bigint/bigint.mojo) [1] [2]
  • Updated method names from to_str to to_string for consistency. (src/decimojo/bigint/bigint.mojo) [1] [2] [3] [4] [5]
  • Removed commented-out initializers to clean up the code. (src/decimojo/bigint/bigint.mojo)

• BigUInt Enhancements:

  • Added a type alias BUInt for BigUInt to improve code readability. (src/decimojo/biguint/biguint.mojo)
  • Updated method names from to_str to to_string for consistency. (src/decimojo/biguint/biguint.mojo) [1] [2] [3]

Import Adjustments:

• Import Statements:
  • Updated import statements to include BigDecimal and the new alias BUInt. (src/decimojo/__init__.mojo)