Update to the next version of proc-macro2

Depends on dtolnay/quote#73
Depends on alexcrichton/proc-macro2#90
Depends on a new nightly
diff --git a/src/lib.rs b/src/lib.rs
index c459395..1f23701 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -329,9 +329,6 @@
 #[cfg(all(any(feature = "full", feature = "derive"), feature = "printing"))]
 pub use generics::{ImplGenerics, Turbofish, TypeGenerics};
 
-mod ident;
-pub use ident::Ident;
-
 #[cfg(feature = "full")]
 mod item;
 #[cfg(feature = "full")]
@@ -732,7 +729,7 @@
 where
     T: quote::ToTokens + Default,
 {
-    fn to_tokens(&self, tokens: &mut quote::Tokens) {
+    fn to_tokens(&self, tokens: &mut proc_macro2::TokenStream) {
         match *self.0 {
             Some(ref t) => t.to_tokens(tokens),
             None => T::default().to_tokens(tokens),