[decimal][integer] Implement comparison operators for BigDecimal + Improve BigUInt.scale_up_by_power_of_10()

[forfudan]

This pull request introduces a new benchmark for scaling up BigUInt values by powers of 10 and includes various improvements and additions to the BigDecimal class. The most important changes include adding a new benchmark file, updating the main benchmark script to include the new benchmark, and enhancing the BigDecimal class with new methods.

New Benchmark:

• Added bench_bigdecimal_scale_up_by_power_of_10.mojo to benchmark the scale_up_by_power_of_10 function for BigUInt values.

Updates to Benchmark Script:

• Updated benches/bigdecimal/bench.mojo to include the new benchmark for scaling up by powers of 10. [1] [2] [3]

Enhancements to BigDecimal Class:

• Added new alias BigDec for BigDecimal in src/decimojo/bigdecimal/bigdecimal.mojo.
• Introduced augmented arithmetic assignment methods (__iadd__, __isub__, __imul__, __itruediv__) and comparison methods (__gt__, __ge__, __lt__, __le__, __eq__, __ne__) to the BigDecimal class.
• Added new methods for comparison and mathematical operations (compare, compare_absolute, max, min) to the BigDecimal class.

Other Changes:

• Updated import statements in src/decimojo/__init__.mojo to include new aliases for BigDecimal and RoundingMode.