Transformation from float to bigint

At the moment 
```res 
let i = 1
let f = 1.3

let bigI = BigInt.fromInt(i)
let bigF = BigInt.fromFloat(f)
```

is compiled to
```js
var bigI = BigInt(1);

var bigF = BigInt(1.3);

var i = 1;

var f = 1.3;
```

Problem: in JS, `BigInt(x)` is valid only if x is an integer, not a float, so running `var bigF = BigInt(1.3)` throws an error