Skip transform_values when dealing with a Dry::Struct

This brings performance improvements of around 1.4-1.9x according to my
benchmarks. `Hashify.[]` is already part of the implementation of
`.to_h` for a `Dry::Struct`.

Also makes it explocit that we are checking here for a `Dry::Struct`,
not a `Struct` from stdlib.

Author: katafrakt

lib/dry/struct/hashify.rb

@@ -5,11 +5,11 @@ class Struct
     # Helper for {Struct#to_hash} implementation
     module Hashify
       # Converts value to hash recursively
-      # @param [#to_hash, #map, Object] value
+      # @param [#to_hash, #to_ary, Object] value
       # @return [Hash, Array]
       def self.[](value)
-        if value.is_a?(Struct)
-          value.to_h.transform_values { self[_1] }
+        if value.is_a?(::Dry::Struct)
+          value.to_h
         elsif value.respond_to?(:to_hash)
           value.to_hash.transform_values { self[_1] }
         elsif value.respond_to?(:to_ary)