`Matrix{2,3,4}::swap_columns` can trigger undefined behavior for identical indices

The Matrix2::swapcolumns, Matrix3::swapcolumns, and Matrix4::swapcolumns implementations call ptr::swap&mut selfa, &mut selfb. When a == b, these safe APIs create two mutable references to the same matrix column and pass them to ptr::swap. This violates Rust's aliasing rules and can trigger...