Auto merge of #12783 - Wafflespeanut:word_spacing, r=Manishearth

Prefer length and percentage for word spacing

<!-- Please describe your changes on the following line: -->
The goal is to make use of `LengthOrPercentage` for word spacing in `ShapingOptions`, but since it makes use of `f32` which doesn't implement `Hash`, we're going for `NotNan<f32>` from [ordered-float](https://github.com/reem/rust-ordered-float/), which supports hashing. Instead of implementing `Hash` for `LengthOrPercentage` and thereby the inner types like `CSSFloat`, `CalcLengthOrPercentage`, etc., we convert it to `(Au, NotNan<f32>)`.

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors

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- [ ] There are tests for these changes

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components/style/values/computed/mod.rs

@@ -4,6 +4,7 @@
 
 use app_units::Au;
 use euclid::size::Size2D;
+use ordered_float::NotNaN;
 use properties::ComputedValues;
 use std::fmt;
 use super::LocalToCss;
@@ -241,6 +242,15 @@ impl LengthOrPercentage {
             Length(_) | Percentage(_) | Calc(_) => false
         }
     }
+
+    pub fn to_hash_key(&self) -> (Au, NotNaN<f32>) {
+        use self::LengthOrPercentage::*;
+        match *self {
+            Length(l) => (l, NotNaN::new(0.0).unwrap()),
+            Percentage(p) => (Au(0), NotNaN::new(p).unwrap()),
+            Calc(c) => (c.length(), NotNaN::new(c.percentage()).unwrap()),
+        }
+    }
 }
 
 impl fmt::Debug for LengthOrPercentage {