Auto merge of #14296 - Wafflespeanut:either, r=emilio

Some generic impls for Either<Length, T>

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We seem to be using `parse_non_negative` a lot for `LengthOrFoo` types, so it's better to have generic impls (now, and in the future) for such things (assuming that `Length` will always be in the first variant of `Either`).

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

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- [x] These changes do not require tests because it's a refactor

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components/style/values/specified/length.rs

@@ -343,6 +343,13 @@ impl Parse for Length {
     }
 }
 
+impl<T> Either<Length, T> {
+    #[inline]
+    pub fn parse_non_negative_length(input: &mut Parser) -> Result<Either<Length, T>, ()> {
+        Length::parse_internal(input, AllowedNumericType::NonNegative).map(Either::First)
+    }
+}
+
 #[derive(Clone, Debug)]
 pub struct CalcSumNode {
     pub products: Vec<CalcProductNode>,
@@ -946,13 +953,6 @@ impl Parse for LengthOrPercentageOrNone {
 
 pub type LengthOrNone = Either<Length, None_>;
 
-impl LengthOrNone {
-    #[inline]
-    pub fn parse_non_negative(input: &mut Parser) -> Result<LengthOrNone, ()> {
-        Length::parse_internal(input, AllowedNumericType::NonNegative).map(Either::First)
-    }
-}
-
 #[derive(Clone, PartialEq, Copy, Debug)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 pub enum LengthOrPercentageOrAutoOrContent {