Version mismatch between fj and fj-app can cause problems

[hannobraun]

Fornjot models depend on the fj crate to define shapes and to interface with the Fornjot app. fj-app compiles and loads those models at runtime.

It is possible to define a Fornjot model using one version of the fj crate, and load this model with a different version of fj-app. This might work, or it might cause a hard-to-understand error message, or weird behavior, or even a segmentation fault. It would be better to prevent this outright, by embedding the version of the fj crate in the compiled artifact and checking that version in fj-app when the model is loaded. A version mismatch should result in a friendly error message.

We already have code that determines the version in fj-app's build.rs. Maybe this build.rs can be moved to the fj crate, so the version is made available there. I think it needs to go into a pub static, to show up as a symbol in the compiled artifact? However it's done, fj-app already depends on fj, so it can just read the version from there for its current needs, instead of relying on its own build.rs.