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/libstd/lib.rs

@@ -142,7 +142,7 @@ extern crate core;
 #[macro_reexport(vec)]
 extern crate "collections" as core_collections;
 
-extern crate "rand" as core_rand;
+#[allow(deprecated)] extern crate "rand" as core_rand;
 extern crate alloc;
 extern crate unicode;
 extern crate libc;