async I/O API: why strings?

[vega]

What is the reason to use the `string` as the buffer to send data and 
`Future[string]` to receive data asynchronously? And it's the standard for 
asyncdispatch, asyncnet and asyncfile. For example, synchronous API works with 
strings and with data pointers. So, is it related with interoperability of GC 
and async calls? The main problem is that if you need to read/write binary 
data, you must copy it from or to strings.