dyn and :: should have a space between

When given this code:
```
fn foo(_x: &u64) -> Option<&(dyn ::std::error::Error + 'static)> {
    None
}
```
rustfmt 1.2.2 stable removes the space between dyn and ::, resulting in this:
```
fn foo(_x: &u64) -> Option<&(dyn::std::error::Error + 'static)> {
    None
}
```