Do not use a second lookup for the contract in StrictHash assertion

`all?` iterator already provides contract for the `valid?` check. Use
it instead of accessing `contract_hash` again.

lib/contracts/builtin_contracts.rb

@@ -407,8 +407,8 @@ def valid?(arg)
         return false unless arg.is_a?(Hash)
         return false unless arg.keys.sort.eql?(contract_hash.keys.sort)
 
-        contract_hash.all? do |key, _v|
-          contract_hash.key?(key) && Contract.valid?(arg[key], contract_hash[key])
+        contract_hash.all? do |key, contract|
+          contract_hash.key?(key) && Contract.valid?(arg[key], contract)
         end
       end
     end