core: Remove the cast module

This commit revisits the `cast` module in libcore and libstd, and scrutinizes
all functions inside of it. The result was to remove the `cast` module entirely,
folding all functionality into the `mem` module. Specifically, this is the fate
of each function in the `cast` module.

* transmute - This function was moved to `mem`, but it is now marked as
              #[unstable]. This is due to planned changes to the `transmute`
              function and how it can be invoked (see the #[unstable] comment).
              For more information, see RFC 5 and #12898

* transmute_copy - This function was moved to `mem`, with clarification that is
                   is not an error to invoke it with T/U that are different
                   sizes, but rather that it is strongly discouraged. This
                   function is now #[stable]

* forget - This function was moved to `mem` and marked #[stable]

* bump_box_refcount - This function was removed due to the deprecation of
                      managed boxes as well as its questionable utility.

* transmute_mut - This function was previously deprecated, and removed as part
                  of this commit.

* transmute_mut_unsafe - This function doesn't serve much of a purpose when it
                         can be achieved with an `as` in safe code, so it was
                         removed.

* transmute_lifetime - This function was removed because it is likely a strong
                       indication that code is incorrect in the first place.

* transmute_mut_lifetime - This function was removed for the same reasons as
                           `transmute_lifetime`

* copy_lifetime - This function was moved to `mem`, but it is marked
                  `#[unstable]` now due to the likelihood of being removed in
                  the future if it is found to not be very useful.

* copy_mut_lifetime - This function was also moved to `mem`, but had the same
                      treatment as `copy_lifetime`.

* copy_lifetime_vec - This function was removed because it is not used today,
                      and its existence is not necessary with DST
                      (copy_lifetime will suffice).

In summary, the cast module was stripped down to these functions, and then the
functions were moved to the `mem` module.

    transmute - #[unstable]
    transmute_copy - #[stable]
    forget - #[stable]
    copy_lifetime - #[unstable]
    copy_mut_lifetime - #[unstable]

[breaking-change]

src/libnative/io/pipe_unix.rs

@@ -10,7 +10,6 @@
 
 use libc;
 use std::c_str::CString;
-use std::cast;
 use std::intrinsics;
 use std::io;
 use std::mem;
@@ -36,7 +35,7 @@ fn addr_to_sockaddr_un(addr: &CString) -> IoResult<(libc::sockaddr_storage, uint
     assert!(mem::size_of::<libc::sockaddr_storage>() >=
             mem::size_of::<libc::sockaddr_un>());
     let mut storage: libc::sockaddr_storage = unsafe { intrinsics::init() };
-    let s: &mut libc::sockaddr_un = unsafe { cast::transmute(&mut storage) };
+    let s: &mut libc::sockaddr_un = unsafe { mem::transmute(&mut storage) };
 
     let len = addr.len();
     if len > s.sun_path.len() - 1 {