define IO#hash to override Enumerable#hash. fixes #73.

Current implementation of Enumerable#hash calls #each to collect
hash values of enumerated objects (then calculates the hash value
of the Enumerable object).  But IO#each is a method to read lines.
It is not expected that IO#hash reads all lines from the IO object.
Or even worse, program waits for the IO object to be readable if
it is a socket or something cannot be read immediately.

mrblib/io.rb

@@ -114,6 +114,12 @@ class IO
     self
   end
 
+  def hash
+    # We must define IO#hash here because IO includes Enumerable and
+    # Enumerable#hash will call IO#read...
+    self.__id__
+  end
+
   def write(string)
     str = string.is_a?(String) ? string : string.to_s
     return str.size unless str.size > 0