Implement rt::io::stdio

Additionally, this moves the prelude imports of print/println from std::io to
std::rt::io.

Closes #6846
Implement BufferedReader.{read_until, read_line}

These two functions will be useful when replacing various other counterparts
used by std::io consumers.
Move std::run off of std::io

This changes the implementation to instead use rt::io::native::process as well
as an example of using those bindings.
Migrate users of io::fd_t to io::native::file::fd_t
Remove usage of io::FILE_writer
Remove some users of io::file_reader
Make the file::DirectoryInfo trait public

This was just a mistake that it was hidden.