Remove IoFactoryObject for ~IoFactory

This involved changing a fair amount of code, rooted in how we access the local
IoFactory instance. I added a helper method to the rtio module to access the
optional local IoFactory. This is different than before in which it was assumed
that a local IoFactory was *always* present. Now, a separate io_error is raised
when an IoFactory is not present, yet I/O is requested.

src/libstd/str.rs

@@ -1018,7 +1018,6 @@ static TAG_CONT_U8: u8 = 128u8;
 
 /// Unsafe operations
 pub mod raw {
-    use option::{Option, Some};
     use cast;
     use libc;
     use ptr;