Add support for model functions that export nothing

[hannobraun]

Model functions are functions defined in a model's source code, that export an artifact. These functions are called when the model is loaded. Currently, there is only one type of model function, which exports an fj::Shape.

It should be possible to define a model function that exports nothing, and just exists for its side effects. This has no purpose within Fornjot, but given that models are written in Rust and could do basically anything, we should support this kind of flexibility. An example use case would be to export a specific file format that Fornjot itself doesn't support.

This is currently blocked on #804.

[hannobraun]

#804 has been addressed. This is no longer blocked.

[hannobraun]

This issue is no longer applicable. The affected code has been removed. See A New Direction for context.