[decimojo] Use bitcast() for coefficient()

[forfudan]

This pull request includes a significant improvement to the coefficient method in the Decimal struct within the src/decimojo/decimal.mojo file. The changes aim to enhance performance by using a faster bitcast implementation.

Enhancements to coefficient method:

• Updated the method documentation to include code formatting for better readability.
• Implemented a fast version of the coefficient method using bitcast to directly convert the three 32-bit parts to a UInt128, given little-endian memory order.
• Commented out the previous arithmetic implementation of combining the three 32-bit parts into a single Int128, retaining it as an alternative.