[decimal] Update comparison functions + code reorganization

[forfudan]

This pull request includes significant changes to the decimojo package, primarily focusing on reorganizing and enhancing the comparison operations for the Decimal type. The changes include adding a new comparison module, updating the Decimal struct to use this module, and modifying related benchmarks and tests.

Key Changes:

Comparison Operations:

• Introduced a new comparison module implementing comparison operations for the Decimal type, including functions like compare, greater, less, greater_equal, less_equal, equal, and not_equal (src/decimojo/comparison.mojo).
• Updated the Decimal struct to use the new comparison module for comparison operations, replacing previous logic-based comparisons (src/decimojo/decimal.mojo).

Code Reorganization:

• Moved comparison functions from the logic module to the new comparison module and updated import statements accordingly (src/decimojo/__init__.mojo, src/decimojo/decimal.mojo, src/decimojo/maths/__init__.mojo).

Benchmarks and Tests:

• Added new benchmarks for from_string and comparison to the benches/bench.mojo file and updated the main function to include these benchmarks (benches/bench.mojo).
• Updated the mojoproject.toml file to include new test and benchmark commands for comparison (mojoproject.toml).