Discard leftover input when consumed (#76)

* Discard leftover input when consumed

The iterator given to `parse_event` now `take()` the value from
`leftover`.

* Always save leftover

`parse_event` will consume it when required.

* Remove OptionIterator, directly uses std::option::Iter

src/input.rs

@@ -56,14 +56,14 @@ impl<R: Read> Iterator for Events<R> {
                 c => parse_event(Ok(c), &mut source.bytes()),
             },
             Ok(2) => {
-                if buf[0] != b'\x1B' {
-                    // this is not an escape sequence, but we read two bytes, save the second byte
-                    // for later
-                    self.leftover = Some(buf[1]);
-                }
-
-                let mut iter = buf[1..2].iter().map(|c| Ok(*c)).chain(source.bytes());
+                let mut option_iter = &mut Some(buf[1]).into_iter();
+                let result = {
+                    let mut iter = option_iter.map(|c| Ok(c)).chain(source.bytes());
                     parse_event(Ok(buf[0]), &mut iter)
+                };
+                // If the option_iter wasn't consumed, keep the byte for later.
+                self.leftover = option_iter.next();
+                result
             }
             Ok(_) => unreachable!(),
             Err(e) => Err(e),