Linkify some documentation

Closes #64

src/lib.rs

@@ -1,23 +1,27 @@
-//! A "shim crate" intended to multiplex the `proc_macro` API on to stable Rust.
+//! A "shim crate" intended to multiplex the [`proc_macro`] API on to stable
+//! Rust.
 //!
 //! Procedural macros in Rust operate over the upstream
-//! `proc_macro::TokenStream` type. This type currently is quite conservative
-//! and exposed no internal implementation details. Nightly compilers, however,
-//! contain a much richer interface. This richer interface allows fine-grained
-//! inspection of the token stream which avoids stringification/re-lexing and
-//! also preserves span information.
+//! [`proc_macro::TokenStream`][ts] type. This type currently is quite
+//! conservative and exposed no internal implementation details. Nightly
+//! compilers, however, contain a much richer interface. This richer interface
+//! allows fine-grained inspection of the token stream which avoids
+//! stringification/re-lexing and also preserves span information.
 //!
-//! The upcoming APIs added to `proc_macro` upstream are the foundation for
+//! The upcoming APIs added to [`proc_macro`] upstream are the foundation for
 //! productive procedural macros in the ecosystem. To help prepare the ecosystem
 //! for using them this crate serves to both compile on stable and nightly and
 //! mirrors the API-to-be. The intention is that procedural macros which switch
 //! to use this crate will be trivially able to switch to the upstream
 //! `proc_macro` crate once its API stabilizes.
 //!
 //! In the meantime this crate also has a `nightly` Cargo feature which
-//! enables it to reimplement itself with the unstable API of `proc_macro`.
+//! enables it to reimplement itself with the unstable API of [`proc_macro`].
 //! This'll allow immediate usage of the beneficial upstream API, particularly
 //! around preserving span information.
+//!
+//! [`proc_macro`]: https://doc.rust-lang.org/proc_macro/
+//! [ts]: https://doc.rust-lang.org/proc_macro/struct.TokenStream.html
 
 // Proc-macro2 types in rustdoc of other crates get linked to here.
 #![doc(html_root_url = "https://docs.rs/proc-macro2/0.2.2")]