Add and Test Basic I64 Functions

[enricozb]

Implement and test all basic functions for I64, following the same pattern as the Nat functions. Include bitwise operations.

1. Implement the following operations for I64:

   • All operations found in Data/Nat
   • Bitwise operations (and, or, xor, not, left shift, right shift, unsigned right shift)

2. For each operation:

   • Create a separate file in the Data/I64 directory (e.g., Data/I64/add.agda, Data/I64/mul.agda, etc.).
   • Follow the existing implementation pattern
   • Create test cases covering normal usage and edge cases