Remove incorrect references to _raw stdio functions

Remove incorrect references to _raw stdio functions

std::io does not currently expose the stdin_raw, stdout_raw, or
stderr_raw functions. According to the current plans for stdio (see RFC
#517), raw access will likely be provided using the platform-specific
std::os::{unix,windows} modules. At the moment we don't expose any way
to do this. As such, delete all mention of the _raw functions from the
stdin/stdout/stderr function documentation.

While we're at it, remove a few `pub`s from items that aren't exposed.
This is done just to lessen the confusion experienced by anyone who
looks at the source in an attempt to find the _raw functions.