Tracking issue: returning iterators from trait methods

[LukasKalbertodt]

Returning iterators from trait methods is difficult as (a) usually every implementing type has a different iterator type and (b) this iterator type borrows from the collection. This is currently not (easily) expressible in Rust. The situation will be vastly improved by GATs. impl Trait for trait methods would make it even easier.

Until those features are implemented, we have to work around the problem. This is done in a number of different ways:

• Mesh::{faces, vertices, edges}: here, there exists only one iterator type which uses the next_*_handle and last_*_handle methods of the Mesh trait. This means we have extracted the iterator logic into two "normal" functions in the trait. This only works because the iterator state is the same for all implementing types: a handle as the current "position" is sufficient. So this workaround doesn't work for situations where different implementors need different iterator state or the iterator logic is more complex.

• Adjacency queries of meshes: here, a funky type system trick was used to work around the lack of GATs. It produces a bunch of verbose filler code, but it works. Each implementor has its own type.

• PropStore: here, dynamic dispatch is used for now. Currently there is no urgent need to improve the speed of this iteration. This workaround could probably be replaced by the one of the adjacency queries, but it's not worth it yet.

[LukasKalbertodt]

The latter two have been resolved by using GATs. The first mentioned one is a bit trickier. I opened #32 for it.