Allow free type of partition subdivisions

[milibopp]

Consider the Partition trait:

pub trait Partition<T>: Sized {
    fn subdivide(&self) -> Vec<Self>;
    /* ... */
}

One could imagine impls of Partition that subdivide into another implementation of Partition. Currently this would have to be worked around by wrapping such partitions in an enum that comprises both. However the following does not compile:

pub trait Partition<T>: Sized {
    type Subdivision: Partition<T>;
    fn subdivide(&self) -> Vec<Subdivision>;
    /* ... */
}

For this to work rust-lang/rust#20551 needs to be fixed (only if such recursion is intended to be allowed, of course).

[milibopp]

With the reformed partition traits the above of course applies to Subdivide instead of Partition.

[milibopp]

This should be possible now, as the aforementioned compiler bug has been fixed.

[milibopp]

Given rust-lang/rust#22446, this is not particularly useful to implement currently. The split between Subdivide and Partition is a problem as one has to provide the information that a subdivided part is a partition in a where clause, and this information does not propagate properly.