Speed up Hash lookups with constants

Since we are always looking up these hashes with an identical duplicate (not just the same value, the same object) we can take advantage of Hash.compare_by_identity.

Suggested by @dgynn in #383 (comment)

lib/sprockets/digest_utils.rb

@@ -80,6 +80,9 @@ def detect_digest_class(bytes)
         digest << val.to_s
       }
     end
+
+    ADD_VALUE_TO_DIGEST.compare_by_identity.rehash
+
     ADD_VALUE_TO_DIGEST.default_proc = ->(_, val) {
       raise TypeError, "couldn't digest #{ val }"
     }