Handle binding overshadowing in function scopes

[ThierryBleau]

let x = 1;
let y = 2;

let f = (x, y) => {
    let x = ();
    let y = [];
    (x,y); 
};
f(1);

Returns type error when unifying TConst("int") and TConst("unit").

We do have expected behavior outside of function scopes however.

let x = 1;
let x = "hello";

x;

Returns x : TConst("string:)