bpo-43244: Rename pycore_ast.h functions to _PyAST_xxx() (GH-25252)

Rename AST functions of pycore_ast.h to use the "_PyAST_" prefix.
Remove macros creating aliases without prefix. For example, Module()
becomes _PyAST_Module(). Update Grammar/python.gram to use
_PyAST_xxx() functions.
diff --git a/Python/ast_opt.c b/Python/ast_opt.c
index 46dba76..311e0c7 100644
--- a/Python/ast_opt.c
+++ b/Python/ast_opt.c
@@ -451,8 +451,9 @@ astfold_body(asdl_stmt_seq *stmts, PyArena *ctx_, _PyASTOptimizeState *state)
             return 0;
         }
         asdl_seq_SET(values, 0, st->v.Expr.value);
-        expr_ty expr = _Py_JoinedStr(values, st->lineno, st->col_offset,
-                                     st->end_lineno, st->end_col_offset, ctx_);
+        expr_ty expr = _PyAST_JoinedStr(values, st->lineno, st->col_offset,
+                                        st->end_lineno, st->end_col_offset,
+                                        ctx_);
         if (!expr) {
             return 0;
         }