Merge: More abstract classes

The current leniency of abstract methods (see #808) make that a lot of concrete classes inherit abstract methods (I counted 5048 cases in the whole code base).
Some trivial cases are classes that just should be abstract.

Pull-Request: #1622
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>

diff --cc src/parser/parser_nodes.nit
Simple merge