Split SassNumber into separate subclasses based on units (#1221)

This allows us to use more efficient implementations in the common
case where numbers don't have complex units.

See #113

Author: nex3

CHANGELOG.md

@@ -1,3 +1,7 @@
+## 1.32.7
+
+* Improve the performance of unitless and single-unit numbers.
+
 ## 1.32.6
 
 ### Node JS API

lib/src/value/external/number.dart

@@ -61,8 +61,8 @@ abstract class SassNumber extends Value {
 
   /// Creates a number with full [numeratorUnits] and [denominatorUnits].
   factory SassNumber.withUnits(num value,
-      {Iterable<String> numeratorUnits,
-      Iterable<String> denominatorUnits}) = internal.SassNumber.withUnits;
+      {List<String> numeratorUnits,
+      List<String> denominatorUnits}) = internal.SassNumber.withUnits;
 
   /// Returns [value] as an [int], if it's an integer value according to
   /// [isInt].