Handle zero-Sierra variables #130

Sometimes, especially in simple functions, we have assignments that produce no Sierra statements. For example, the function:

fn foo(x: felt252) -> felt252 {
    let y = x;
    0 
}

has Sierra:

f1:
drop<felt252>([0]);
const_as_immediate<felt252, 0>() -> [1];
return([1]);

the binding is not hittable and y does not exist in Sierra, which makes y impossible to show