Refactor: Move BPY mathutils headers to proper C++ ones.

This module actually was confusing, since only a few headers were
effectively still C ones (using the `extern "C"` blocks)...

source/blender/python/mathutils/mathutils_Euler.hh

@@ -0,0 +1,39 @@
+/* SPDX-FileCopyrightText: 2023 Blender Authors
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later */
+
+#pragma once
+
+/** \file
+ * \ingroup pymathutils
+ */
+
+extern PyTypeObject euler_Type;
+#define EulerObject_Check(v) PyObject_TypeCheck((v), &euler_Type)
+#define EulerObject_CheckExact(v) (Py_TYPE(v) == &euler_Type)
+
+struct EulerObject {
+  BASE_MATH_MEMBERS(eul);
+  unsigned char order; /* rotation order */
+};
+
+/* struct data contains a pointer to the actual data that the
+ * object uses. It can use either PyMem allocated data (which will
+ * be stored in py_data) or be a wrapper for data allocated through
+ * blender (stored in blend_data). This is an either/or struct not both */
+
+/* prototypes */
+
+PyObject *Euler_CreatePyObject(const float eul[3],
+                               short order,
+                               PyTypeObject *base_type) ATTR_WARN_UNUSED_RESULT;
+PyObject *Euler_CreatePyObject_wrap(float eul[3],
+                                    short order,
+                                    PyTypeObject *base_type) ATTR_WARN_UNUSED_RESULT
+    ATTR_NONNULL(1);
+PyObject *Euler_CreatePyObject_cb(PyObject *cb_user,
+                                  short order,
+                                  unsigned char cb_type,
+                                  unsigned char cb_subtype) ATTR_WARN_UNUSED_RESULT;
+
+short euler_order_from_string(const char *str, const char *error_prefix);