* 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
