[decimal] Remove str._float_to_decimal_str() + Add bench for from_string

[forfudan]

This pull request introduces comprehensive benchmarks for the Decimal string constructor operations in the benches/bench_from_string.mojo file and removes the decimojo.str module. The key changes are grouped into two themes: benchmarking and code cleanup.

Benchmarking:

• Added a new file benches/bench_from_string.mojo to benchmark the performance of the Decimal string constructor against Python's decimal module across diverse test cases. This includes functions to open a log file, print messages to both console and log file, run benchmarks for different cases, and display a summary of results.

Code Cleanup:

• Removed the import statement for decimojo.str from src/decimojo/decimal.mojo as it is no longer needed.
• Deleted the src/decimojo/str.mojo file, which contained string manipulation functions for the Decimal type. These functions are no longer used.