tracer: fix STORE_DEREF handling, add EXTENDED_ARG support.

This fixes the following issues:

- on Python 3.10 and earlier, storing to free variables is now handled
  correctly
- on Python 3.11, `_varname_from_oparg` is now used, fixing problems
  with cell variables that are also arguments
- on all supported versions, EXTENDED_ARG is now parsed, ensuring proper
  handling for long functions

Fixes #792.

tests/test_tracer.py

@@ -0,0 +1,79 @@
+from amaranth.hdl.ast import *
+from types import SimpleNamespace
+
+from .utils import *
+
+class TracerTestCase(FHDLTestCase):
+    def test_fast(self):
+        s1 = Signal()
+        self.assertEqual(s1.name, "s1")
+        s2 = Signal()
+        self.assertEqual(s2.name, "s2")
+
+    def test_name(self):
+        class Dummy:
+            s1 = Signal()
+            self.assertEqual(s1.name, "s1")
+            s2 = Signal()
+            self.assertEqual(s2.name, "s2")
+
+    def test_attr(self):
+        ns = SimpleNamespace()
+        ns.s1 = Signal()
+        self.assertEqual(ns.s1.name, "s1")
+        ns.s2 = Signal()
+        self.assertEqual(ns.s2.name, "s2")
+
+    def test_index(self):
+        l = [None]
+        l[0] = Signal()
+        self.assertEqual(l[0].name, "$signal")
+
+    def test_deref_cell(self):
+        s1 = Signal()
+        self.assertEqual(s1.name, "s1")
+        s2 = Signal()
+        self.assertEqual(s2.name, "s2")
+
+        def dummy():
+            return s1, s2
+
+    def test_deref_free(self):
+        def inner():
+            nonlocal s3, s4
+            s3 = Signal()
+            s4 = Signal()
+            return s1, s2
+
+        s1 = Signal()
+        s2 = Signal()
+        s3 = None
+        s4 = None
+        inner()
+        self.assertEqual(s1.name, "s1")
+        self.assertEqual(s2.name, "s2")
+        self.assertEqual(s3.name, "s3")
+        self.assertEqual(s4.name, "s4")
+
+    def test_long(self):
+        test = ""
+        for i in range(100000):
+            test += f"dummy{i} = None\n"
+        test += "s1 = Signal()\n"
+        test += "s2 = Signal()\n"
+        ns = {"Signal": Signal}
+        exec(test, ns)
+        self.assertEqual(ns["s1"].name, "s1")
+        self.assertEqual(ns["s2"].name, "s2")
+
+    def test_deref_fast(self):
+        def inner(s2):
+            s1 = Signal()
+            s2 = Signal()
+            self.assertEqual(s1.name, "s1")
+            self.assertEqual(s2.name, "s2")
+
+            def dummy():
+                return s1, s2
+
+        inner(None)