improve length/2 (#1325)

build/instructions_template.rs

@@ -538,6 +538,8 @@ enum SystemClauseType {
     PopCount,
     #[strum_discriminants(strum(props(Arity = "1", Name = "$cpu_now")))]
     CpuNow,
+    #[strum_discriminants(strum(props(Arity = "2", Name = "$det_length_rundown")))]
+    DeterministicLengthRundown,
     REPL(REPLCodePtr),
 }
 
@@ -1658,6 +1660,7 @@ fn generate_instruction_preface() -> TokenStream {
                     &Instruction::CallInstallNewBlock(_) |
                     &Instruction::CallMaybe(_) |
                     &Instruction::CallCpuNow(_) |
+                    &Instruction::CallDeterministicLengthRundown(_) |
                     &Instruction::CallCurrentTime(_) |
                     &Instruction::CallQuotedToken(_) |
                     &Instruction::CallReadTermFromChars(_) |
@@ -1860,6 +1863,7 @@ fn generate_instruction_preface() -> TokenStream {
                     &Instruction::ExecuteInstallNewBlock(_) |
                     &Instruction::ExecuteMaybe(_) |
                     &Instruction::ExecuteCpuNow(_) |
+                    &Instruction::ExecuteDeterministicLengthRundown(_) |
                     &Instruction::ExecuteCurrentTime(_) |
                     &Instruction::ExecuteQuotedToken(_) |
                     &Instruction::ExecuteReadTermFromChars(_) |