Change syntax of where-clauses on associated types

[dtolnay]

rust-lang/rust#89122, which is in FCP, proposes that:

type StringMap<K> where K: PartialOrd = BTreeMap<K, String>;

should instead be spelled as:

type StringMap<K> = BTreeMap<K, String>
where
    K: PartialOrd;

If the proposal passes, we'll need to implement this in syn.