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/blenkernel/intern/customdata.cc

@@ -19,7 +19,6 @@
 
 #include "BLI_bitmap.h"
 #include "BLI_color.hh"
-#include "BLI_cpp_type_make.hh"
 #include "BLI_endian_switch.h"
 #include "BLI_index_range.hh"
 #include "BLI_math.h"
@@ -5407,5 +5406,3 @@ size_t CustomData_get_elem_size(CustomDataLayer *layer)
 {
   return LAYERTYPEINFO[layer->type].size;
 }
-
-BLI_CPP_TYPE_MAKE(MStringProperty, MStringProperty, CPPTypeFlags::None);