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/libsyntax/attr.rs

@@ -340,14 +340,14 @@ pub fn cfg_matches(diagnostic: &SpanHandler, cfgs: &[P<MetaItem>], cfg: &ast::Me
 }
 
 /// Represents the #[deprecated="foo"] and friends attributes.
-#[deriving(Encodable,Decodable,Clone,Show)]
+#[deriving(RustcEncodable,RustcDecodable,Clone,Show)]
 pub struct Stability {
     pub level: StabilityLevel,
     pub text: Option<InternedString>
 }
 
 /// The available stability levels.
-#[deriving(Copy,Encodable,Decodable,PartialEq,PartialOrd,Clone,Show)]
+#[deriving(RustcEncodable,RustcDecodable,PartialEq,PartialOrd,Clone,Show,Copy)]
 pub enum StabilityLevel {
     Deprecated,
     Experimental,
@@ -464,7 +464,7 @@ fn int_type_of_word(s: &str) -> Option<IntType> {
     }
 }
 
-#[deriving(Copy, PartialEq, Show, Encodable, Decodable)]
+#[deriving(PartialEq, Show, RustcEncodable, RustcDecodable, Copy)]
 pub enum ReprAttr {
     ReprAny,
     ReprInt(Span, IntType),
@@ -483,7 +483,7 @@ impl ReprAttr {
     }
 }
 
-#[deriving(Copy, Eq, Hash, PartialEq, Show, Encodable, Decodable)]
+#[deriving(Eq, Hash, PartialEq, Show, RustcEncodable, RustcDecodable, Copy)]
 pub enum IntType {
     SignedInt(ast::IntTy),
     UnsignedInt(ast::UintTy)