Improve XLS C++ synthesis-only fixed-point and integer library correctness and performance.

[copybara-service]

Improve XLS C++ synthesis-only fixed-point and integer library correctness and performance.

This CL optimizes and fixes several issues in the xls_fixed and xls_int synthesis-only headers:

• Replaces per-bit loops with slice operations to prevent O(W^2) IR node generation during synthesis.
• Fixes operand width calculations for division to correctly handle signed/unsigned mismatches.
• Implements the modulo (%) and modulo-assign (%=) operators for XlsInt.
• Restricts implicit conversions to 64-bit built-in types for XlsInt types wider than 64 bits.
• Adds support for set_val() and ac::init_array() to initialize fixed-point and integer arrays with special values (zero, quantum, min, max).
• Adds shift operators taking plain int offsets.
• Adds comprehensive unit tests covering rounding, saturation, division, modulo, and array initialization.