[fine] Assignments!

And new error capabilities!

fine/src/vm.rs

@@ -161,6 +161,15 @@ impl Frame {
         }
     }
 
+    fn store_argument(&mut self, i: usize, v: StackValue) -> Result<()> {
+        if i >= self.locals.len() {
+            Err(VMErrorCode::LocalOutOfRange(i))
+        } else {
+            self.args[i] = v;
+            Ok(())
+        }
+    }
+
     fn pop_function(&mut self) -> Result<FuncValue> {
         match self.pop_value()? {
             StackValue::Function(f) => Ok(FuncValue::Function(f)),
@@ -236,6 +245,11 @@ fn eval_one(
         Instruction::Discard => {
             f.pop_value()?;
         }
+        Instruction::Dup => {
+            let v = f.pop_value()?;
+            f.push_value(v.clone());
+            f.push_value(v);
+        }
         Instruction::FloatAdd => {
             let x = f.pop_float()?;
             let y = f.pop_float()?;
@@ -300,6 +314,10 @@ fn eval_one(
         Instruction::PushTrue => {
             f.push_bool(true);
         }
+        Instruction::StoreArgument(i) => {
+            let v = f.pop_value()?;
+            f.store_argument(i, v)?;
+        }
         Instruction::StoreLocal(i) => {
             let v = f.pop_value()?;
             f.store_local(i, v)?;