Remove more warnings.

Mostly of the 'unused imports' kind.

src/libcore/rt/stack.rs

@@ -39,9 +39,7 @@ pub impl StackSegment {
 
     /// Point to the low end of the allocated stack
     fn start(&self) -> *uint {
-        unsafe {
-            vec::raw::to_ptr(self.buf) as *uint
-        }
+      vec::raw::to_ptr(self.buf) as *uint
     }
 
     /// Point one word beyond the high end of the allocated stack

src/libcore/rt/uv/mod.rs

@@ -44,9 +44,6 @@ use vec;
 use ptr;
 use cast;
 use str;
-use option::*;
-use str::raw::from_c_str;
-use to_str::ToStr;
 use libc::{c_void, c_int, size_t, malloc, free};
 use cast::transmute;
 use ptr::null;

src/libcore/rt/uv/uvio.rs

@@ -11,7 +11,6 @@
 use option::*;
 use result::*;
 use ops::Drop;
-use old_iter::CopyableIter;
 use cell::{Cell, empty_cell};
 use cast::transmute;
 use clone::Clone;

src/libcore/unstable/lang.rs

@@ -16,7 +16,6 @@ use libc::{c_char, c_uchar, c_void, size_t, uintptr_t, c_int, STDERR_FILENO};
 use managed::raw::BoxRepr;
 use str;
 use sys;
-use cast::transmute;
 use rt::{context, OldTaskContext};
 use rt::local_services::borrow_local_services;
 use option::{Option, Some, None};