Implement size_hint for more iterators.

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

components/style/font_face.rs

@@ -170,6 +170,10 @@ impl Iterator for EffectiveSources {
     fn next(&mut self) -> Option<Source> {
         self.0.pop()
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (self.0.len(), Some(self.0.len()))
+    }
 }
 
 struct FontFaceRuleParser<'a, 'b: 'a> {

components/style/properties/declaration_block.rs

@@ -99,6 +99,10 @@ impl<'a> Iterator for DeclarationImportanceIterator<'a> {
         self.iter.next().map(|(decl, important)|
             (decl, if important { Importance::Important } else { Importance::Normal }))
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.iter.size_hint()
+    }
 }
 
 impl<'a> DoubleEndedIterator for DeclarationImportanceIterator<'a> {

components/style/stylesheet_set.rs

@@ -45,6 +45,10 @@ where
     fn next(&mut self) -> Option<Self::Item> {
         self.0.next().map(|entry| &entry.sheet)
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.0.size_hint()
+    }
 }
 
 /// The validity of the data in a given cascade origin.

components/style/values/computed/mod.rs

@@ -198,6 +198,10 @@ impl<'a, 'cx, 'cx_a: 'cx, S: ToComputedValue + 'a> Iterator for ComputedVecIter<
             None
         }
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (self.values.len(), Some(self.values.len()))
+    }
 }
 
 /// A trait to represent the conversion between computed and specified values.