bpo-40334: Improve various PEG-Parser related stuff (GH-19669)

The changes in this commit are all related to @vstinner's original review comments of the initial PEP 617 implementation PR.
diff --git a/Include/internal/pegen_interface.h b/Include/internal/pegen_interface.h
new file mode 100644
index 0000000..d8621c1
--- /dev/null
+++ b/Include/internal/pegen_interface.h
@@ -0,0 +1,34 @@
+#ifndef Py_PEGENINTERFACE
+#define Py_PEGENINTERFACE
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#ifndef Py_BUILD_CORE
+#  error "this header requires Py_BUILD_CORE define"
+#endif
+
+#include "Python.h"
+#include "Python-ast.h"
+
+PyAPI_FUNC(mod_ty) PyPegen_ASTFromFile(const char *filename, int mode, PyArena *arena);
+PyAPI_FUNC(mod_ty) PyPegen_ASTFromString(const char *str, int mode, PyCompilerFlags *flags,
+                                         PyArena *arena);
+PyAPI_FUNC(mod_ty) PyPegen_ASTFromStringObject(const char *str, PyObject* filename, int mode,
+                                               PyCompilerFlags *flags, PyArena *arena);
+PyAPI_FUNC(mod_ty) PyPegen_ASTFromFileObject(FILE *fp, PyObject *filename_ob,
+                                             int mode, const char *enc, const char *ps1,
+                                             const char *ps2, int *errcode, PyArena *arena);
+PyAPI_FUNC(PyCodeObject *) PyPegen_CodeObjectFromFile(const char *filename, int mode);
+PyAPI_FUNC(PyCodeObject *) PyPegen_CodeObjectFromString(const char *str, int mode,
+                                                        PyCompilerFlags *flags);
+PyAPI_FUNC(PyCodeObject *) PyPegen_CodeObjectFromFileObject(FILE *, PyObject *filename_ob,
+                                                            int mode, const char *enc,
+                                                            const char *ps1,
+                                                            const char *ps2,
+                                                            int *errcode);
+
+#ifdef __cplusplus
+}
+#endif
+#endif /* !Py_PEGENINTERFACE*/