nitc: renamed --no-check-initialization in --no-check-attr-isset to follow the new specification of nullable attributes

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
nitc: add option --count-isset-checks to count all compiled isset checks on non-nullable attributes

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
benches: add benches about nullables things

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>