bpo-43244: Add pycore_compile.h header file (GH-25000)

Remove the compiler functions using "struct _mod" type, because the
public AST C API was removed:

* PyAST_Compile()
* PyAST_CompileEx()
* PyAST_CompileObject()
* PyFuture_FromAST()
* PyFuture_FromASTObject()

These functions were undocumented and excluded from the limited C API.

Rename functions:

* PyAST_CompileObject() => _PyAST_Compile()
* PyFuture_FromASTObject() => _PyFuture_FromAST()

Moreover, _PyFuture_FromAST() is no longer exported (replace
PyAPI_FUNC() with extern). _PyAST_Compile() remains exported for
test_peg_generator.

Remove also compatibility functions:

* PyAST_Compile()
* PyAST_CompileEx()
* PyFuture_FromAST()

Include/internal/pycore_compile.h

@@ -0,0 +1,40 @@
+#ifndef Py_INTERNAL_COMPILE_H
+#define Py_INTERNAL_COMPILE_H
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#ifndef Py_BUILD_CORE
+#  error "this header requires Py_BUILD_CORE define"
+#endif
+
+struct _mod;   // Type defined in pycore_ast.h
+
+// Export the symbol for test_peg_generator (built as a library)
+PyAPI_FUNC(PyCodeObject*) _PyAST_Compile(
+    struct _mod *mod,
+    PyObject *filename,
+    PyCompilerFlags *flags,
+    int optimize,
+    PyArena *arena);
+extern PyFutureFeatures* _PyFuture_FromAST(
+    struct _mod * mod,
+    PyObject *filename
+    );
+
+extern PyObject* _Py_Mangle(PyObject *p, PyObject *name);
+
+typedef struct {
+    int optimize;
+    int ff_features;
+} _PyASTOptimizeState;
+
+extern int _PyAST_Optimize(
+    struct _mod *,
+    PyArena *arena,
+    _PyASTOptimizeState *state);
+
+#ifdef __cplusplus
+}
+#endif
+#endif /* !Py_INTERNAL_COMPILE_H */