serialize: Fully deprecate the library

This commit completes the deprecation story for the in-tree serialization
library. The compiler will now emit a warning whenever it encounters
`deriving(Encodable)` or `deriving(Decodable)`, and the library itself is now
marked `#[unstable]` for when feature staging is enabled.

All users of serialization can migrate to the `rustc-serialize` crate on
crates.io which provides the exact same interface as the libserialize library
in-tree. The new deriving modes are named `RustcEncodable` and `RustcDecodable`
and require `extern crate "rustc-serialize" as rustc_serialize` at the crate
root in order to expand correctly.

To migrate all crates, add the following to your `Cargo.toml`:

    [dependencies]
    rustc-serialize = "0.1.1"

And then add the following to your crate root:

    extern crate "rustc-serialize" as rustc_serialize;

Finally, rename `Encodable` and `Decodable` deriving modes to `RustcEncodable`
and `RustcDecodable`.

[breaking-change]

src/libtest/lib.rs

@@ -37,6 +37,7 @@
 extern crate getopts;
 extern crate regex;
 extern crate serialize;
+extern crate "serialize" as rustc_serialize;
 extern crate term;
 
 pub use self::TestFn::*;
@@ -213,7 +214,7 @@ pub struct TestDescAndFn {
     pub testfn: TestFn,
 }
 
-#[deriving(Clone, Copy, Encodable, Decodable, PartialEq, Show)]
+#[deriving(Clone, RustcEncodable, RustcDecodable, PartialEq, Show, Copy)]
 pub struct Metric {
     value: f64,
     noise: f64