bpo-41746: Add type information to asdl_seq objects (GH-22223)

* Add new capability to the PEG parser to type variable assignments. For instance:
```
       | a[asdl_stmt_seq*]=';'.small_stmt+ [';'] NEWLINE { a }
```

* Add new sequence types from the asdl definition (automatically generated)
* Make `asdl_seq` type a generic aliasing pointer type.
* Create a new `asdl_generic_seq` for the generic case using `void*`.
* The old `asdl_seq_GET`/`ast_seq_SET` macros now are typed.
* New `asdl_seq_GET_UNTYPED`/`ast_seq_SET_UNTYPED` macros for dealing with generic sequences.
* Changes all possible `asdl_seq` types to use specific versions everywhere.

Python/future.c

@@ -13,11 +13,10 @@ static int
 future_check_features(PyFutureFeatures *ff, stmt_ty s, PyObject *filename)
 {
     int i;
-    asdl_seq *names;
 
     assert(s->kind == ImportFrom_kind);
 
-    names = s->v.ImportFrom.names;
+    asdl_alias_seq *names = s->v.ImportFrom.names;
     for (i = 0; i < asdl_seq_LEN(names); i++) {
         alias_ty name = (alias_ty)asdl_seq_GET(names, i);
         const char *feature = PyUnicode_AsUTF8(name->name);