Simplify the node_eff representation as well as change the names

used in order to make it homogoneous with what is done in SyntaxTreeCore.ml

This commit is related to the previous one actually.

Also remove all this story of node_half_eff that is not
used (yet), and that may not be useful (we'll see later).

Also continue to fix the representation of SyntaxTrreCore.node_info :
 -> remove the node alias
 -> put the corresponding infomation in node_body field
 -> rename node_body field into node_def
 -> associate to node_def (instead of a body option) a new union
type made of Abstract, Extern, Alias of ..., Body of ...

This allows us to
 - remove an "assert false" to deal with node with body and alias
(this new presentation makes it impossible)
 - Deal with Abstract node properly