Switch to add/remove instruction mutation

gen/opcode.go

@@ -48,7 +48,7 @@ var opCodes = []vm.OpCodeType{
 	vm.OpSqrt,
 }
 
-func randOpCode() vm.OpCodeType {
+func RandOpCode() vm.OpCodeType {
 	// Uniform distribution
 	return opCodes[rand.Intn(len(opCodes))]
 }