[decimal] simplifying initialization of Decimal and add from_uint128()

[forfudan]

This pull request includes significant refactoring and improvements to the Decimal struct in the decimojo library. The main changes involve simplifying the initialization of Decimal objects and adding a new method for creating Decimal instances from UInt128 values.

Initialization Refactoring:

• Simplified the initialization logic by removing redundant constructors and consolidating the initialization process for Int and UInt128 values.

New Method for UInt128 Initialization:

• Added a new static method from_uint128 to initialize a Decimal from a UInt128 value, including validation for the value and scale.

Code Simplification:

• Updated existing methods to use the new from_uint128 method for creating Decimal instances from UInt128 values.
• Modified the sqrt function to use the new from_uint128 method.
• Updated the round function to use the new from_uint128 method.