Pretty-printers support discrete vectors and matrices.
Add toplevel pretty-printers
Simplify the implementation of pretty-printers
Add function `Mat.init' as an alias of `Mat.init_cols'

- Slap.Mat.init
- Slap.[SDCZ].Mat.init
Modify pretty printers

- Change the name of type `line_type' to `index' (Arguments of constructors `Head' and `Foot' are removed)
- Change the names of several local variables
- Improve documentation
Merge pretty-printers