[integer][decimal] Allows implicit type conversion and type merging

[forfudan]

This pull request introduces significant updates to BigDecimal, BigInt, and BigUInt structures in the decimojo library. The changes add implicit constructors for better usability, refine initialization methods, and improve type handling for integral and floating-point scalars.

Enhancements to BigDecimal:

• src/decimojo/bigdecimal/bigdecimal.mojo: Added implicit constructors for BigUInt, BigInt, Int, and integral scalars, enabling seamless initialization of BigDecimal objects from these types.
• src/decimojo/bigdecimal/bigdecimal.mojo: Replaced the from_scalar method with from_integral_scalar and introduced from_float for better handling of integral and floating-point scalars. Improved constraints and error handling for scalar type validation. [1] [2]
• src/decimojo/bigdecimal/bigdecimal.mojo: Added right-side arithmetic dunders (__radd__, __rsub__, etc.) for BigDecimal, enabling more flexible cross-type operations.

Enhancements to BigInt:

• src/decimojo/bigint/bigint.mojo: Added implicit constructors for BigUInt, Int, and integral scalars, simplifying the initialization of BigInt objects. [1] [2]
• src/decimojo/bigint/bigint.mojo: Introduced from_integral_scalar for handling integral scalars, replacing from_uint128 with improved constraints and validation.

Minor Refinements:

• src/decimojo/bigdecimal/exponential.mojo: Updated the sqrt function to use BigUInt.from_unsigned_integral_scalar, ensuring consistent handling of unsigned integral scalars.
• src/decimojo/bigint/bigint.mojo: Marked certain initialization methods as unsafe and added warnings in their documentation to encourage users to use safer alternatives. [1] [2]

These changes collectively improve the usability, readability, and robustness of the decimojo library, making it more intuitive for developers working with arbitrary-precision arithmetic.