Change syntax of where-clauses on associated types

https://github.com/rust-lang/rust/issues/89122, which is in FCP, proposes that:

```rust
type StringMap<K> where K: PartialOrd = BTreeMap<K, String>;
```

should instead be spelled as:

```rust
type StringMap<K> = BTreeMap<K, String>
where
    K: PartialOrd;
```

If the proposal passes, we'll need to implement this in syn.