Fix no-arg dir(), convert it to new args style

tests/snippets/builtin_dir.py

@@ -1,3 +1,5 @@
+assert isinstance(dir(), list)
+assert '__builtins__' in dir()
 
 class A:
 	def test():
@@ -21,6 +23,16 @@ assert ['h', 'q'] == dir(B())
 # This calls type.__dir__ so isn't changed (but inheritance works)!
 assert 'test' in dir(A)
 
+# eval() takes any mapping-like type, so dir() must support them
+# TODO: eval() should take any mapping as locals, not just dict-derived types
+class A(dict):
+	def __getitem__(self, x):
+		return dir
+	def keys(self):
+		yield 6
+		yield 5
+assert eval("dir()", {}, A()) == [5, 6]
+
 import socket
 
 assert "AF_INET" in dir(socket)

vm/src/builtins.rs

@@ -111,15 +111,13 @@ fn builtin_delattr(obj: PyObjectRef, attr: PyStringRef, vm: &VirtualMachine) ->
     vm.del_attr(&obj, attr.into_object())
 }
 
-fn builtin_dir(vm: &VirtualMachine, args: PyFuncArgs) -> PyResult {
-    if args.args.is_empty() {
-        Ok(vm.get_locals().into_object())
-    } else {
-        let obj = args.args.into_iter().next().unwrap();
-        let seq = vm.call_method(&obj, "__dir__", vec![])?;
-        let sorted = builtin_sorted(vm, PyFuncArgs::new(vec![seq], vec![]))?;
-        Ok(sorted)
-    }
+fn builtin_dir(obj: OptionalArg<PyObjectRef>, vm: &VirtualMachine) -> PyResult {
+    let seq = match obj {
+        OptionalArg::Present(obj) => vm.call_method(&obj, "__dir__", vec![])?,
+        OptionalArg::Missing => vm.call_method(&vm.get_locals().into_object(), "keys", vec![])?,
+    };
+    let sorted = builtin_sorted(vm, PyFuncArgs::new(vec![seq], vec![]))?;
+    Ok(sorted)
 }
 
 fn builtin_divmod(vm: &VirtualMachine, args: PyFuncArgs) -> PyResult {
@@ -136,6 +134,7 @@ fn builtin_divmod(vm: &VirtualMachine, args: PyFuncArgs) -> PyResult {
 /// Implements `eval`.
 /// See also: https://docs.python.org/3/library/functions.html#eval
 fn builtin_eval(vm: &VirtualMachine, args: PyFuncArgs) -> PyResult {
+    // TODO: support any mapping for `locals`
     arg_check!(
         vm,
         args,