MatchDataValue must be a class

Negatable value modifiers don't get updated when they're structs.

src/spectator/matchers/alternative_match_data_value.cr

@@ -3,7 +3,7 @@ require "./match_data_value"
 module Spectator::Matchers
   # Selects a value based on whether the value is negated.
   # This is used when a matcher is negated.
-  private struct AlternativeMatchDataValue(T1, T2) < MatchDataValue
+  private class AlternativeMatchDataValue(T1, T2) < MatchDataValue
     # Creates the wrapper.
     def initialize(@value1 : T1, @value2 : T2)
       @negated = false

src/spectator/matchers/generic_match_data_value.cr

@@ -2,7 +2,7 @@ require "./match_data_value"
 
 module Spectator::Matchers
   # Wraps a value for used in match data.
-  private struct GenericMatchDataValue(T) < MatchDataValue
+  private class GenericMatchDataValue(T) < MatchDataValue
     # Creates the wrapper.
     def initialize(@value : T)
     end

src/spectator/matchers/match_data_value.cr

@@ -1,6 +1,6 @@
 module Spectator::Matchers
   # Abstract base for all match data values.
   # All sub-classes are expected to implement their own `#to_s`.
-  private abstract struct MatchDataValue
+  private abstract class MatchDataValue
   end
 end

src/spectator/matchers/negatable_match_data_value.cr

@@ -3,7 +3,7 @@ require "./match_data_value"
 module Spectator::Matchers
   # Wraps an expected value that can be negated.
   # This is used when a matcher is negated.
-  private struct NegatableMatchDataValue(T) < MatchDataValue
+  private class NegatableMatchDataValue(T) < MatchDataValue
     # Negatable value.
     getter value
 

src/spectator/matchers/negatable_prefixed_match_data_value.cr

@@ -3,7 +3,7 @@ require "./match_data_value"
 module Spectator::Matchers
   # Wraps a prefixed value that can be negated.
   # This is used when a matcher is negated.
-  private struct NegatablePrefixedMatchDataValue(T) < MatchDataValue
+  private class NegatablePrefixedMatchDataValue(T) < MatchDataValue
     # Negatable value.
     getter value
 

src/spectator/matchers/prefixed_match_data_value.cr

@@ -2,7 +2,7 @@ require "./match_data_value"
 
 module Spectator::Matchers
   # Prefixes (for output formatting) an actual or expected value.
-  private struct PrefixedMatchDataValue(T) < MatchDataValue
+  private class PrefixedMatchDataValue(T) < MatchDataValue
     # Value being prefixed.
     getter value : T