NekoML: Improvement on Type Inference

Not really a bug, just improvement :smile:

```typescript
type node {
Number : (float)
Add : (node, node)
Multiply: (node, node)
}

function rec eval(node) { // Awesome.nml(11): Cannot unify int and float
match node {
| Number(value) -> value // this is float
| Add(left, right) -> eval(left) + eval(right)
| Multiply(left, right) -> eval(left) * eval(right)
}
}

var node = Add(Number(1.0), Multiply(Number(2.0), Number(3.0)));

print(eval(node));
```