add IO#rewind

mrblib/io.rb

@@ -141,6 +141,10 @@ class IO
     seek(i, SEEK_SET)
   end
 
+  def rewind
+    seek(0, SEEK_SET)
+  end
+
   def seek(i, whence = SEEK_SET)
     raise IOError if closed?
     @pos = sysseek(i, whence)

test/io.rb

@@ -256,6 +256,17 @@ assert('IO#pos=, IO#seek') do
   io.closed?
 end
 
+assert('IO#rewind') do
+  fd = IO.sysopen $mrbtest_io_rfname
+  io = IO.new fd
+  assert_equal 'm', io.getc
+  assert_equal 1, io.pos
+  assert_equal 0, io.rewind
+  assert_equal 0, io.pos
+  io.close
+  io.closed?
+end
+
 assert('IO#gets') do
   fd = IO.sysopen $mrbtest_io_rfname
   io = IO.new fd