py/compile: Raise an error on async with/for outside an async function.

A simple reproducer is:

   async for x in (): x

Before this change, it would cause an assertion error in mpy-cross and
micropython-coverage.

tests/basics/async_syntaxerror.py

@@ -0,0 +1,19 @@
+# test syntax errors using async
+
+try:
+    exec
+except NameError:
+    print("SKIP")
+    raise SystemExit
+
+
+def test_syntax(code):
+    try:
+        exec(code)
+        print("no SyntaxError")
+    except SyntaxError:
+        print("SyntaxError")
+
+
+test_syntax("async for x in (): x")
+test_syntax("async with x: x")