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/libcoretest/lib.rs

@@ -12,6 +12,7 @@
 #![feature(int_uint)]
 #![feature(unboxed_closures)]
 #![feature(unsafe_destructor, slicing_syntax)]
+#![allow(deprecated)] // rand
 
 extern crate core;
 extern crate test;