I started working on #465 and made some corresponding std.io
API changes.

New structs:
 * std.io.FileInStream
 * std.io.FileOutStream
 * std.io.BufferedOutStream
 * std.io.BufferedInStream

Removed:
 * std.io.File.in_stream
 * std.io.File.out_stream

Now instead of &file.out_stream or &file.in_stream to get access to
the stream API for a file, you get it like this:

var file_in_stream = io.FileInStream.init(&file);
const in_stream = &file_in_stream.stream;

var file_out_stream = io.FileOutStream.init(&file);
const out_stream = &file_out_stream.stream;

This is evidence that we might not need any OOP features -
See #130.