libcore: Remove a few possibly-cyclic imports in an effort to unbreak the tree on Linux

src/libcore/io.rs

@@ -1204,9 +1204,11 @@ pub fn read_whole_file(file: &Path) -> Result<~[u8], ~str> {
 // fsync related
 
 pub mod fsync {
-    use prelude::*;
     use io::{FILERes, FdRes, fd_t};
+    use kinds::Copy;
     use libc;
+    use ops::Drop;
+    use option::{None, Option, Some};
     use os;
 
     pub enum Level {

src/libcore/libc.rs

@@ -230,8 +230,6 @@ pub mod types {
                 pub type uintptr_t = uint;
             }
             pub mod posix88 {
-                use prelude::*;
-
                 pub type off_t = i32;
                 pub type dev_t = u64;
                 pub type ino_t = u32;