std: Second pass stabilization for ptr

This commit performs a second pass for stabilization over the `std::ptr` module.
The specific actions taken were:

* The `RawPtr` trait was renamed to `PtrExt`
* The `RawMutPtr` trait was renamed to `MutPtrExt`
* The module name `ptr` is now stable.
* These functions were all marked `#[stable]` with no modification:
  * `null`
  * `null_mut`
  * `swap`
  * `replace`
  * `read`
  * `write`
  * `PtrExt::is_null`
  * `PtrExt::offset`
* These functions remain unstable:
  * `as_ref`, `as_mut` - the return value of an `Option` is not fully expressive
                         as null isn't the only bad value, and it's unclear
                         whether we want to commit to these functions at this
                         time. The reference/lifetime semantics as written are
                         also problematic in how they encourage arbitrary
                         lifetimes.
  * `zero_memory` - This function is currently not used at all in the
                    distribution, and in general it plays a broader role in the
                    "working with unsafe pointers" story. This story is not yet
                    fully developed, so at this time the function remains
                    unstable for now.
  * `read_and_zero` - This function remains unstable for largely the same
                      reasons as `zero_memory`.
* These functions are now all deprecated:
  * `PtrExt::null` - call `ptr::null` or `ptr::null_mut` instead.
  * `PtrExt::to_uint` - use an `as` expression instead.
  * `PtrExt::is_not_null` - use `!p.is_null()` instead.

src/libstd/sys/common/net.rs

@@ -269,7 +269,7 @@ pub fn get_host_addresses(host: Option<&str>, servname: Option<&str>,
     // Collect all the results we found
     let mut addrs = Vec::new();
     let mut rp = res;
-    while rp.is_not_null() {
+    while !rp.is_null() {
         unsafe {
             let addr = try!(sockaddr_to_addr(mem::transmute((*rp).ai_addr),
                                              (*rp).ai_addrlen as uint));

src/libstd/sys/unix/backtrace.rs

@@ -244,7 +244,7 @@ fn print(w: &mut Writer, idx: int, addr: *mut libc::c_void) -> IoResult<()> {
     use iter::{Iterator, IteratorExt};
     use os;
     use path::GenericPath;
-    use ptr::RawPtr;
+    use ptr::PtrExt;
     use ptr;
     use slice::SliceExt;