Replace custom_encodable with encodable.

By default, `newtype_index!` types get a default `Encodable`/`Decodable`
impl. You can opt out of this with `custom_encodable`. Opting out is the
opposite to how Rust normally works with autogenerated (derived) impls.

This commit inverts the behaviour, replacing `custom_encodable` with
`encodable` which opts into the default `Encodable`/`Decodable` impl.
Only 23 of the 59 `newtype_index!` occurrences need `encodable`.

Even better, there were eight crates with a dependency on
`rustc_serialize` just from unused default `Encodable`/`Decodable`
impls. This commit removes that dependency from those eight crates.

compiler/rustc_ast/src/ast.rs

@@ -2574,7 +2574,6 @@ pub enum AttrStyle {
 }
 
 rustc_index::newtype_index! {
-    #[custom_encodable]
     #[debug_format = "AttrId({})"]
     pub struct AttrId {}
 }

compiler/rustc_ast/src/node_id.rs

@@ -8,6 +8,7 @@ rustc_index::newtype_index! {
     /// This is later turned into [`DefId`] and `HirId` for the HIR.
     ///
     /// [`DefId`]: rustc_span::def_id::DefId
+    #[encodable]
     #[debug_format = "NodeId({})"]
     pub struct NodeId {
         /// The [`NodeId`] used to represent the root of the crate.