[std::vec] Rename .pop_opt() to .pop(), drop the old .pop() behavior

src/libstd/io/process.rs

@@ -161,8 +161,11 @@ impl Drop for Process {
     fn drop(&mut self) {
         // Close all I/O before exiting to ensure that the child doesn't wait
         // forever to print some text or something similar.
-        for _ in range(0, self.io.len()) {
-            self.io.pop();
+        loop {
+            match self.io.pop() {
+                Some(_) => (),
+                None => break,
+            }
         }
 
         self.wait();