Auto merge of #18494 - frewsxcv:frewsxcv-size-hint, r=jdm

Implement `size_hint` for more iterators.

```
implement size hint for
more iterators because why
not we like fast things
```

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components/script/dom/htmlselectelement.rs

@@ -440,4 +440,12 @@ impl<I, J, K, T> Iterator for Choice3<I, J, K>
             Choice3::Third(ref mut k) => k.next(),
         }
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        match *self {
+            Choice3::First(ref i) => i.size_hint(),
+            Choice3::Second(ref j) => j.size_hint(),
+            Choice3::Third(ref k) => k.size_hint(),
+        }
+    }
 }

components/script/dom/servoparser/mod.rs

@@ -487,6 +487,10 @@ impl<I> Iterator for FragmentParsingResult<I>
         next.remove_self();
         Some(next)
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.inner.size_hint()
+    }
 }
 
 #[derive(HeapSizeOf, JSTraceable, PartialEq)]