`tree<y>.calculatePatch(otherTree<x>)` types could be improved, so that it returns a `Patch<Y | X>`

[stefanpenner]

When using calculatePatch today, the resulting patches incorrectly type their entry as DefaultEntry, when ideally the resulting Patch[] would be generic based on the actually tree's being used. Basically something like: Patch<T | K>[]

today:

class FSTree<T> {
  calculatePatch<K extends BaseEntry>(theirFSTree: FSTree<K>, isEqual?: (a: T, b: K) => boolean): FSTree.Patch {
  
  }
}

Proposed:

class FSTree<T> {
  calculatePatch<K extends BaseEntry>(theirFSTree: FSTree<K>, isEqual?: (a: T, b: K) => boolean): FSTree.Patch<T | K> {
  
  }
}