Convert newtype_index to a proc macro

The `macro_rules!` implementation was becomng excessively complicated,
and difficult to modify. The new proc macro implementation should make
it much easier to add new features (e.g. skipping certain `#[derive]`s)

Author: Aaron1011

compiler/rustc_index/src/lib.rs

@@ -9,3 +9,5 @@
 pub mod bit_set;
 pub mod interval;
 pub mod vec;
+
+pub use rustc_macros::newtype_index;