add test func for scanf

Signed-off-by: etienne <etienne.basset@ensta.org>
add a parse_conversion_specifier function

Signed-off-by: etienne <etienne.basset@ensta.org>
move conversion specifier parsing out of st_vscanf so we can reuse it

Signed-off-by: etienne <etienne.basset@ensta.org>
add 'ap' argument to match_expr family
end goal will be that somthing like "(%W )?" or ("%IP )? is permitted and do the right thing

Signed-off-by: etienne <etienne.basset@ensta.org>