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/parse/token.rs

@@ -28,7 +28,7 @@ use std::path::BytesContainer;
 use std::rc::Rc;
 
 #[allow(non_camel_case_types)]
-#[deriving(Clone, Copy, Encodable, Decodable, PartialEq, Eq, Hash, Show)]
+#[deriving(Clone, RustcEncodable, RustcDecodable, PartialEq, Eq, Hash, Show, Copy)]
 pub enum BinOpToken {
     Plus,
     Minus,
@@ -43,7 +43,7 @@ pub enum BinOpToken {
 }
 
 /// A delimeter token
-#[deriving(Clone, Copy, Encodable, Decodable, PartialEq, Eq, Hash, Show)]
+#[deriving(Clone, RustcEncodable, RustcDecodable, PartialEq, Eq, Hash, Show, Copy)]
 pub enum DelimToken {
     /// A round parenthesis: `(` or `)`
     Paren,
@@ -53,14 +53,14 @@ pub enum DelimToken {
     Brace,
 }
 
-#[deriving(Clone, Copy, Encodable, Decodable, PartialEq, Eq, Hash, Show)]
+#[deriving(Clone, RustcEncodable, RustcDecodable, PartialEq, Eq, Hash, Show, Copy)]
 pub enum IdentStyle {
     /// `::` follows the identifier with no whitespace in-between.
     ModName,
     Plain,
 }
 
-#[deriving(Clone, Copy, Encodable, Decodable, PartialEq, Eq, Hash, Show)]
+#[deriving(Clone, RustcEncodable, RustcDecodable, PartialEq, Eq, Hash, Show, Copy)]
 pub enum Lit {
     Byte(ast::Name),
     Char(ast::Name),
@@ -86,7 +86,7 @@ impl Lit {
 }
 
 #[allow(non_camel_case_types)]
-#[deriving(Clone, Encodable, Decodable, PartialEq, Eq, Hash, Show)]
+#[deriving(Clone, RustcEncodable, RustcDecodable, PartialEq, Eq, Hash, Show)]
 pub enum Token {
     /* Expression-operator symbols. */
     Eq,
@@ -334,7 +334,7 @@ impl Token {
     }
 }
 
-#[deriving(Clone, Encodable, Decodable, PartialEq, Eq, Hash)]
+#[deriving(Clone, RustcEncodable, RustcDecodable, PartialEq, Eq, Hash)]
 /// For interpolation during macro expansion.
 pub enum Nonterminal {
     NtItem(P<ast::Item>),