std: Tweak String implementations

This commit performs a pass over the implementations of the new `String` trait
in the formatting module. Some implementations were removed as a conservative
move pending an upcoming convention about `String` implementations, and some
were added in order to retain consistency across the libraries. Specifically:

* All "smart pointers" implement `String` now, adding missing implementations
  for `Arc` and `Rc`.
* The `Vec<T>` and `[T]` types no longer implement `String`.
* The `*const T` and `*mut T` type no longer implement `String`.
* The `()` type no longer implements `String`.
* The `Path` type's `Show` implementation does not surround itself with `Path
  {}` (a minor tweak).

All implementations of `String` in this PR were also marked `#[stable]` to
indicate that the types will continue to implement the `String` trait regardless
of what it looks like.

src/libstd/path/posix.rs

@@ -60,7 +60,7 @@ pub fn is_sep(c: char) -> bool {
 
 impl fmt::Show for Path {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f, "Path {{ {} }}", self.display())
+        fmt::Show::fmt(&self.display(), f)
     }
 }
 

src/libstd/path/windows.rs

@@ -87,7 +87,7 @@ pub struct Path {
 
 impl fmt::Show for Path {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f, "Path {{ {} }}", self.display())
+        fmt::Show::fmt(&self.display(), f)
     }
 }