async def syntax rigor and __await__ magic method

Some examples of improved compliance with CPython that currently
have divergent behavior in CircuitPython are listed below:

* yield from is not allowed in async methods
```
>>> async def f():
...     yield from 'abc'
...
Traceback (most recent call last):
  File "<stdin>", line 2, in f
SyntaxError: 'yield from' inside async function
```

* await only works on awaitable expressions
```
>>> async def f():
...     await 'not awaitable'
...
>>> f().send(None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in f
AttributeError: 'str' object has no attribute '__await__'
```

* only __await__()able expressions are awaitable
Okay this one actually does not work in circuitpython at all today.
This is how CPython works though and pretending __await__ does not
exist will only bite users who write both.
```
>>> class c:
...     pass
...
>>> def f(self):
...     yield
...     yield
...     return 'f to pay respects'
...
>>> c.__await__ = f  # could just as easily have put it on the class but this shows how it's wired
>>> async def g():
...     awaitable_thing = c()
...     partial = await awaitable_thing
...     return 'press ' + partial
...
>>> q = g()
>>> q.send(None)
>>> q.send(None)
>>> q.send(None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration: press f to pay respects
```

1 files changed, 8 insertions, 1 deletions

diff --git a/py/compile.c b/py/compile.c
index 653aae004..f19430298 100644
--- a/py/compile.c
+++ b/py/compile.c
@@ -2632,6 +2632,12 @@ STATIC void compile_yield_expr(compiler_t *comp, mp_parse_node_struct_t *pns) {
         EMIT_ARG(yield, MP_EMIT_YIELD_VALUE);
     } else if (MP_PARSE_NODE_IS_STRUCT_KIND(pns->nodes[0], PN_yield_arg_from)) {
         pns = (mp_parse_node_struct_t*)pns->nodes[0];
+#if MICROPY_PY_ASYNC_AWAIT
+        if(comp->scope_cur->scope_flags & MP_SCOPE_FLAG_ASYNC) {
+            compile_syntax_error(comp, (mp_parse_node_t)pns, translate("'yield from' inside async function"));
+            return;
+        }
+#endif
         compile_node(comp, pns->nodes[0]);
         compile_yield_from(comp);
     } else {
@@ -2648,7 +2654,8 @@ STATIC void compile_atom_expr_await(compiler_t *comp, mp_parse_node_struct_t *pn
     }
     compile_require_async_context(comp, pns);
     compile_atom_expr_normal(comp, pns);
-    compile_yield_from(comp);
+
+    compile_await_object_method(comp, MP_QSTR___await__);
 }
 #endif