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_lint/src/levels.rs

@@ -56,7 +56,6 @@ struct LintLevelSets {
 }
 
 rustc_index::newtype_index! {
-    #[custom_encodable] // we don't need encoding
     struct LintStackIndex {
         const COMMAND_LINE = 0;
     }