More systematically organize code handling interactions between various matrix types?

Code handling interactions between special matrix types (such as `Diagonal`), between annotation types (such as `LowerTriangular`), and between special matrix and annotation types exists distributed throughout base/linalg.

To illustrate, the name and comment at the top of base/linalg/special.jl indicates that that file contains code handling interactions between special matrix types. But that file also contains code handling interactions between special matrix and annotation types, for example `convert(::Type{UnitLowerTriangular}, A::Diagonal)`. And immediately related methods, for example `convert(::Type{LowerTriangular}, A::Diagonal)`, appear in base/linalg/diagonal.jl.

Would more systematically organizing code handling those interactions be worthwhile at this stage?

For example, would {introducing a file like base/linalg/special.jl but dedicated to handling interactions between special matrix and annotation types} and {segregrating corresponding code in that file} be a step forward? Would similarly {introducing a file equivalent to base/linalg/special.jl for annotation types} and {segregating corresponding code in that file} be a step forward?

Thoughts? Best!

Start by comparing the interaction methods in base/linalg/special.jl and base/linalg/diagonal.jl, including conversions involving special and annotation matrix types. Determine whether separate files would improve the organization; done means reaching an agreed structure and scope for any reorganization.

Written by the indexing model from the issue text.