Deprecate in-tree rand, std::rand and #[derive(Rand)].

Use the crates.io crate `rand` (version 0.1 should be a drop in
replacement for `std::rand`) and `rand_macros` (`#[derive_Rand]` should
be a drop-in replacement).

[breaking-change]

src/libsyntax/ext/deriving/rand.rs

@@ -24,6 +24,10 @@ pub fn expand_deriving_rand<F>(cx: &mut ExtCtxt,
                                push: F) where
     F: FnOnce(P<Item>),
 {
+    cx.span_warn(span,
+                 "`#[derive(Rand)]` is deprecated in favour of `#[derive_Rand]` from \
+                  `rand_macros` on crates.io");
+
     let trait_def = TraitDef {
         span: span,
         attributes: Vec::new(),