Move library crates under 'lib/'.

Give these crates each a more standard directory layout with sources in
a 'src' sub-sirectory and Cargo.toml in the top lib/foo directory.

Add license and description fields to each.

The build script for the cretonne crate now lives in
'lib/cretonne/build.rs' separating it from the normal library sources
under 'lib/cretonne/src'.

lib/reader/src/error.rs

@@ -0,0 +1,44 @@
+//! Define the `Location`, `Error`, and `Result` types.
+
+#![macro_use]
+
+use std::fmt;
+use std::result;
+
+/// The location of a `Token` or `Error`.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+pub struct Location {
+    pub line_number: usize,
+}
+
+/// A parse error is returned when the parse failed.
+#[derive(Debug)]
+pub struct Error {
+    pub location: Location,
+    pub message: String,
+}
+
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "{}: {}", self.location.line_number, self.message)
+    }
+}
+
+pub type Result<T> = result::Result<T, Error>;
+
+// Create an `Err` variant of `Result<X>` from a location and `format!` args.
+macro_rules! err {
+    ( $loc:expr, $msg:expr ) => {
+        Err($crate::Error {
+            location: $loc.clone(),
+            message: String::from($msg),
+        })
+    };
+
+    ( $loc:expr, $fmt:expr, $( $arg:expr ),+ ) => {
+        Err($crate::Error {
+            location: $loc.clone(),
+            message: format!( $fmt, $( $arg ),+ ),
+        })
+    };
+}