Auto merge of #12918 - hsinewu:12902-typedefs, r=Manishearth

12902 typedefs

<!-- Please describe your changes on the following line: -->
Using type alias instead of newtype.
Removing duplicated implementation, it's already inherited.
No more type constructors and foo.0
---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #12902 (github issue number if applicable).

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- [X] These changes do not require tests because it's refactoring.

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components/style/properties/helpers/animated_properties.mako.rs

@@ -496,13 +496,6 @@ impl Interpolate for Position {
     }
 }
 
-impl Interpolate for BackgroundPosition {
-    #[inline]
-    fn interpolate(&self, other: &Self, time: f64) -> Result<Self, ()> {
-        Ok(BackgroundPosition(try!(self.0.interpolate(&other.0, time))))
-    }
-}
-
 impl Interpolate for BackgroundSize {
     fn interpolate(&self, other: &Self, time: f64) -> Result<Self, ()> {
         use properties::longhands::background_size::computed_value::ExplicitSize;