RCR 111: IO subclass for string

I'd like to see a sub-class of IO that used a String for internal storage.

Something like:

a = IOString::new
a.puts "Hello, world!"
str = a.string # maybe a.to_s ?
puts str
b = IOString::open(str)
str = b.gets
puts str

I use this kind of structure to store parts of files in place, for parsing later... I created my own class to do this (which was easy, since I knew exactly which methods of IO I'd be talking to), but it seems like something that belongs in the standard library.