implement dynamic key; implement __curPos; other small changes

fix/src/lib.rs

@@ -113,7 +113,7 @@ impl Evaluator {
         _expr: &str,
         _scope: &mut HashSet<StringId>,
     ) -> Result<fix_common::Value> {
-        todo!()
+        todo!("add_binding")
     }
 
     pub fn compile_bytecode(&mut self, source: Source) -> Result<InstructionPtr> {
@@ -248,7 +248,7 @@ impl<'a, R: VmRuntimeCtx> BytecodeContext for CompilerCtx<'a, R> {
             Float(x) => StaticValue::new_float(x),
             Bool(x) => StaticValue::new_inline(x),
             String(x) => StaticValue::new_inline(x),
-            Path(_) => todo!(),
+            Path(_) => todo!("path value type"),
             PrimOp {
                 id,
                 arity,