some missing macros / convenience functions

[DeliciousHair]

Is your feature request related to a problem? Please describe. It makes sense to have the macros col! and row! in order to be consistent with mat!. This also seems a good opportunity to include a related macro (or function) blockwise!:

blockwise![[A, B], [C, D]];

// ┌───────┬──┐
// │       │  │
// │   A   │ B│
// │       │  │
// ├───────┼──┤
// │   C   │ D│
// └───────┴──┘

[sarah-ek]

solved by #100