Make io::with_str_writer pure

[erickt]

If io::with_str_writer was pure, it could then be used safely with to_str::ToStr.

[erickt]

And with this, we should also make these similar functions pure:

io::with_bytes_reader
io::with_bytes_writer
io::with_str_reader

[catamorphism]

Fixed in b18a151 -- thanks @erickt !