Move stdlib into its own crate

stdlib/src/int.rs

@@ -0,0 +1,20 @@
+/// An i64 that always panics on overflow.
+pub struct Int(i64);
+
+impl Int {
+    pub fn abs(&self) -> Self {
+        let Int(int_self) = self;
+
+        let (output, underflowed) = int_self.overflowing_abs();
+
+        if underflowed {
+            underflow_panic();
+        }
+
+        Int(output)
+    }
+}
+
+fn underflow_panic() -> ! {
+    panic!("Underflow!");
+}