BLI: improve CPPType system

* Support bidirectional type lookups. E.g. finding the base type of a
  field was supported, but not the other way around. This also removes
  the todo in `get_vector_type`. To achieve this, types have to be
  registered up-front.
* Separate `CPPType` from other "type traits". For example, previously
  `ValueOrFieldCPPType` adds additional behavior on top of `CPPType`.
  Previously, it was a subclass, now it just contains a reference to the
  `CPPType` it corresponds to. This follows the composition-over-inheritance
  idea. This makes it easier to have self-contained "type traits" without
  having to put everything into `CPPType`.

Differential Revision: https://developer.blender.org/D16479

source/blender/functions/FN_init.h

@@ -0,0 +1,13 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+
+#pragma once
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+void FN_register_cpp_types(void);
+
+#ifdef __cplusplus
+}
+#endif