[types] Clean up Pair and Sym types

Reconfigure the top-level object types.
- Obj is an enum pointer type
- Object is a trait that all Scheme types should implement

Define Sym, a symbol type.
Define Pair, a pair/cons/list type.

Implement Display for all types above. Implement casting methods for the above.

types/src/pair.rs

@@ -0,0 +1,41 @@
+/* types/src/pair.rs
+ * Eryn Wells <eryn@erynwells.me>
+ */
+
+use std::any::Any;
+use std::fmt;
+use super::*;
+use object::Object;
+
+pub struct Pair {
+    car: Obj,
+    cdr: Obj
+}
+
+impl Pair {
+    fn fmt_pair(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        let r = write!(f, "{}", self.car);
+        r.and_then(|r| match self.cdr {
+            Obj::Null => Ok(r),  // Don't write anything.
+            Obj::Ptr(ref next) => {
+                match next.as_pair() {
+                    Some(next_pair) => write!(f, " ").and_then(|_| next_pair.fmt_pair(f)),
+                    None => write!(f, " . {}", next)
+                }
+            }
+        })
+    }
+}
+
+impl Object for Pair {
+    fn as_any(&self) -> &Any { self }
+    fn as_pair(&self) -> Option<&Pair> { Some(self) }
+    fn as_sym(&self) -> Option<&Sym> { None }
+}
+
+impl fmt::Display for Pair {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "(").and_then(|_| self.fmt_pair(f))
+                      .and_then(|_| write!(f, ")"))
+    }
+}