Bump version, upgrade bootstrap

This commit updates the version number to 1.17.0 as we're not on that version of
the nightly compiler, and at the same time this updates src/stage0.txt to
bootstrap from freshly minted beta compiler and beta Cargo.

src/liballoc/boxed.rs

@@ -293,7 +293,6 @@ impl<T: ?Sized> Box<T> {
     }
 }
 
-#[cfg(not(stage0))]
 #[stable(feature = "rust1", since = "1.0.0")]
 unsafe impl<#[may_dangle] T: ?Sized> Drop for Box<T> {
     fn drop(&mut self) {

src/liballoc/heap.rs

@@ -143,14 +143,6 @@ unsafe fn exchange_malloc(size: usize, align: usize) -> *mut u8 {
     }
 }
 
-#[cfg(not(test))]
-#[cfg(stage0)]
-#[lang = "exchange_free"]
-#[inline]
-unsafe fn exchange_free(ptr: *mut u8, old_size: usize, align: usize) {
-    deallocate(ptr, old_size, align);
-}
-
 #[cfg(not(test))]
 #[lang = "box_free"]
 #[inline]