io.rb: add `IO#readbyte`; ref #5389

mrbgems/mruby-io/README.md

@@ -77,7 +77,7 @@ Add the line below to your build configuration.
 | IO#puts                    |    o     |      |
 | IO#read                    |    o     |      |
 | IO#read_nonblock           |          |      |
-| IO#readbyte                |          |      |
+| IO#readbyte                |    o     |      |
 | IO#readchar                |    o     |      |
 | IO#readline                |    o     |      |
 | IO#readlines               |    o     |      |

mrbgems/mruby-io/mrblib/io.rb

@@ -299,6 +299,11 @@ class IO
     end
   end
 
+  def readbyte
+    _read_buf
+    IO._bufread(@buf, 1).getbyte(0)
+  end
+
   def getbyte
     ret = getc
     return ret.getbyte 0 if ret