Move derive internals into serde_derive crate

We can continue to publish serde_derive_internals independently but
serde_derive no longer has a dependency on it. This improves compile
time of serde_derive by 7%.

serde_derive_internals/lib.rs

@@ -0,0 +1,23 @@
+// Copyright 2017 Serde Developers
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![doc(html_root_url = "https://docs.rs/serde_derive_internals/0.23.1")]
+#![cfg_attr(
+    feature = "cargo-clippy",
+    allow(cyclomatic_complexity, doc_markdown, match_same_arms, redundant_field_names)
+)]
+
+#[macro_use]
+extern crate syn;
+
+extern crate proc_macro2;
+
+#[path = "src/mod.rs"]
+mod internals;
+
+pub use internals::*;