Add support for experimental features

[hannobraun]

It should be possible mark specific features as "experimental", for the purpose of communicating to users how solid they should expect specific features to be, and to define a stable subset. Using experimental features should require some kind of opt-in.

Notes on requirements:

• Since Fornjot is intended to be language-agnostic (with Rust just being the currently supported modeling language), opting into experimental features can't be based on Rust language features, and can't happen at compile-time.
• It should be possible to opt-into experimental features per-model, possibly even finer-grained. A component used in a model might use a known-working subset of an experimental feature, but that doesn't mean the author of the model wants to opt into that feature.

Notes on implementation:

• I believe the best place to handle experimental features is the fj-operations crate. The kernel itself is probably not suitable for this, as the concepts it uses don't map well to user-facing concepts.
• Which subset of a model opts into which experimental feature should probably be tracked through flags within the fj data structures. I haven't worked out how exactly to do this, but I'm sure a solution can be found.

This issue replaces #46.

[hannobraun]

Closing as out of scope. See A New Direction for context.