Add Slap.Io.Context module for compatibility with Lacaml

- Slap.Io.default_ellipsis -> Slap.Io.Context.ellipsis_default
- Slap.Io.default_max_rows -> Slap.Io.vertical_default
- Slap.Io.default_max_cols -> Slap.Io.horizontal_default
- Slap.Io.set_max -> Slap.Io.set_dim_defaults
- Optional arguments `max_rows' and `max_cols' of pp_table and so on are
  renamed to `vertical_context' and `horizontal_context', respectively.
Add `ssc' and `lsc' to set the current contexts on toplevel
Improve _oasis