Fix 1 : Fix copyright year
Fix 2 : fix copyright year
Fix 1 : change symbol type to be a record.
Fix 1 : move functions on ident to the Ident module
fix 1 : Move instanciations of IdentSet/Map/Tbl to the Ident module

Also Drop Lambda.IdentSet module and rename every use to Ident.Set
Fix 1 : Change style of Flambda.data