ndarray has a history of encoding arrays’ dimensionality in the type system; this turns out to be useful both for debugging and for writing complex array libraries. However, some arrays don’t (or can’t) have their dimensionality known at compile time. One good example of this is the output of ArrayBase::squeeze: even if the array’s dimensionality is known before the operation, the dimensionality of the output depends on how many axes have length one.

The Dimensionality trait is intended to unify both the known- and unknown-dimensionality cases. Compile-time dimensionalities are represented using NDim, while cases where the dimensionality cannot be known at compile time are represented using DDyn.

A key design choice is that there is no way to recover the number of dimensions as a runtime value from a type implementing Dimensionality. A dynamic dimensionality here does not mean “known at runtime”, but rather “cannot be known at compile time”. The actual number of axes is always taken from the array’s shape, which avoids having two separate sources of truth.

…ions (#1575)

This is the next PR to address #1506. In the previous PR (#1568) we
established a trait for capturing the "dimensionality" or "rank" of an
array at the type level. This PR is focused on providing a bridge from
type-level dimensionality to runtime dimensionality, which we do through
the new `Ranked` trait.

The `Ranked` trait is pretty simple: require an associated type (`NDim`)
that carries compile-time dimensionality, and a function `fn ndim(&self)
-> usize` that tells the user runtime dimensionality. The
`src/layout/ranked.rs` file contains the definition, implementations,
and blanket implementations.

This commit also does some moves and renames of the existing
`Dimensionality` trait, renaming it to `Rank`, as well as many of
the types in that file.