style: Move LengthPercentage to its own file.

I'm (sadly) about to make it a bit more complicated to pack it better. So we
may as well do this so it is easier to reason about navigate.

I also reordered things a bit, and removed some From<> implementations and
such.

Differential Revision: https://phabricator.services.mozilla.com/D58701

components/style/values/specified/position.rs

@@ -297,7 +297,7 @@ impl<S: Side> ToComputedValue for PositionComponent<S> {
                 let p = Percentage(1. - length.percentage());
                 let l = -length.unclamped_length();
                 // We represent `<end-side> <length>` as `calc(100% - <length>)`.
-                ComputedLengthPercentage::new_calc(l, p)
+                ComputedLengthPercentage::new_calc(l, Some(p))
             },
             PositionComponent::Side(_, Some(ref length)) |
             PositionComponent::Length(ref length) => length.to_computed_value(context),