feat: support TraceLowLevelEvent::DropVariable in write_trace()

Author: nickysn

runtime_tracing/src/capnptrace.rs

@@ -419,12 +419,15 @@ pub fn write_trace(q: &[crate::TraceLowLevelEvent], output: &mut impl std::io::W
                     q.set_i(vars[i].0.try_into().unwrap());
                 }
             }
+            TraceLowLevelEvent::DropVariable(varid) => {
+                let mut ret = event.init_drop_variable();
+                ret.set_i(varid.0.try_into().unwrap());
+            }
             TraceLowLevelEvent::CompoundValue(cvr) => todo!(),
             TraceLowLevelEvent::CellValue(cvr) => todo!(),
             TraceLowLevelEvent::AssignCompoundItem(aci) => todo!(),
             TraceLowLevelEvent::AssignCell(acr) => todo!(),
             TraceLowLevelEvent::VariableCell(vcr) => todo!(),
-            TraceLowLevelEvent::DropVariable(varid) => todo!(),
         }
     }