This refactors `SocketValueVariant` with the following goals in mind:
* Support type erasure so that not all users of `SocketValueVariant` have
to know about all the types sockets can have.
* Move towards supporting "rainbow sockets" which are sockets whoose
type is only known at run-time.
* Reduce complexity when dealing with socket values in general. Previously,
one had to use `SocketValueVariantCPPType` a lot to manage uninitialized
memory. This is better abstracted away now.
One related change that I had to do that I didn't see coming at first was that
I had to refactor `set_default_remaining_outputs` because now the default value
of a `SocketValueVariant` would not contain any value. Previously, it was
initialized the zero-value of the template parameter. Similarly, I had to change
how implicit conversions are created, because comparing the `CPPType` of linked
sockets was not enough anymore to determine if a conversion is necessary.
We could potentially use `SocketValueVariant` for the remaining socket types in the
future as well. Not entirely sure if that helps yet. `SocketValueVariant` can easily be
adapted to make that work though. That would also justify the name
"SocketValueVariant" better.
Pull Request: https://projects.blender.org/blender/blender/pulls/116231
e1d0d70911