Add signals + descriptions

Add various missing signals and their descriptions
nitg: Corrected bug when creating a new AVarExpr, uses the last inferred type instead of its definition type.

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
Merge branch 'reassign_fixes' of https://github.com/R4PaSs/nit
Merge branch 'Patch-signals' of https://github.com/CaptainKali/nit