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/src/internals/ctxt.rs

@@ -0,0 +1,56 @@
+// 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.
+
+use std::cell::RefCell;
+use std::fmt::Display;
+use std::thread;
+
+#[derive(Default)]
+pub struct Ctxt {
+    errors: RefCell<Option<Vec<String>>>,
+}
+
+impl Ctxt {
+    pub fn new() -> Self {
+        Ctxt {
+            errors: RefCell::new(Some(Vec::new())),
+        }
+    }
+
+    pub fn error<T: Display>(&self, msg: T) {
+        self.errors
+            .borrow_mut()
+            .as_mut()
+            .unwrap()
+            .push(msg.to_string());
+    }
+
+    pub fn check(self) -> Result<(), String> {
+        let mut errors = self.errors.borrow_mut().take().unwrap();
+        match errors.len() {
+            0 => Ok(()),
+            1 => Err(errors.pop().unwrap()),
+            n => {
+                let mut msg = format!("{} errors:", n);
+                for err in errors {
+                    msg.push_str("\n\t# ");
+                    msg.push_str(&err);
+                }
+                Err(msg)
+            }
+        }
+    }
+}
+
+impl Drop for Ctxt {
+    fn drop(&mut self) {
+        if !thread::panicking() && self.errors.borrow().is_some() {
+            panic!("forgot to check for errors");
+        }
+    }
+}