Naming arguments in Fn traits

Hi,
Right now If there's a Fn* trait that has 2 parameters of the same type it's really easy to confuse which is which.
i.e.:
```
type Callback = FnMut(&[u8], &[u8]) -> Point;
```
If we could *name the arguments* it would become easier to read and harder to misuse.
```
type Callback = FnMut(x: &[u8], y: &[u8]) -> Point;
```
