Fix memoized subject from context description another way

The other method gave a strange compiler error when running specs.
Something to do with "K" not having an inspect method.

In /usr/share/crystal/src/hash.cr:1826:13

 1826 | key.inspect(io)
            ^------
Error: undefined method 'inspect' for K

src/spectator/dsl/groups.cr

@@ -24,17 +24,7 @@ module Spectator
             {{what}}
           end
 
-          @%wrapper : ::Spectator::TypedValueWrapper({{what}})?
-
-          def subject(*args)
-            if (wrapper = @%wrapper)
-              wrapper.value
-            else
-              described_class.new(*args).tap do |value|
-                @%wrapper = ::Spectator::TypedValueWrapper.new(value)
-              end
-            end
-          end
+          subject { described_class.new }
         {% else %}
           def _spectator_implicit_subject(*args)
             {{what}}