Fold threshold_cpu.py into eval.py

Add order of operations circuit (arithmetic.expr_add_mul)

Add 3-operand adder circuit (arithmetic.add3_8bit)

Add PUSH/POP/RET stack operation tests

Add barrel shifter and priority encoder tests

Fix DEC eval test, add INC/DEC/NEG/ROL/ROR tests

Add SHL, SHR, MUL, DIV, and comparator circuits

Add unified eval.py consolidating iron_eval and comprehensive_eval