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/blenlib/BLI_parameter_pack_utils.hh

@@ -23,6 +23,13 @@ template<typename T, T Element> struct TypeForValue {
   static constexpr T value = Element;
 };
 
+/**
+ * A struct that allows passing in a type as a function parameter.
+ */
+template<typename T> struct TypeTag {
+  using type = T;
+};
+
 /**
  * A type that encodes a list of values of the same type.
  * This is similar to #std::integer_sequence, but a bit more general. It's main purpose it to also