Merge #398: Implement is_empty on TokenIter

cb51a5b Implement is_empty on TokenIter (Tobin C. Harding)

Pull request description:

  Clippy emits:

  ```
  warning: struct `TokenIter` has a public `len` method, but no `is_empty` method
     --> src/miniscript/lex.rs:102:5
      |
  102 |     pub fn len(&self) -> usize {
      |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
      |
      = note: `#[warn(clippy::len_without_is_empty)]` on by default
      = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#len_without_is_empty
  ```

  Iterators typically do not implement `is_empty` because there is often no way for an iterator to know if there are more items without calling `next`, however we use a `Vec` under the hood so `is_empty` is cheap.

  Implement `is_empty` on the `TokenIter` struct.

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Tree-SHA512: b0b832ac9457b9eef725b4589198212d90ea66faa4788792551da31a7601b928edbd7658f33862ed8210478cc4512af4ebb35be1cddf9d66ac67f6222f282a3e

Author: sanket1729

src/miniscript/lex.rs

@@ -102,6 +102,11 @@ impl<'s> TokenIter<'s> {
     pub fn len(&self) -> usize {
         self.0.len()
     }
+
+    /// Returns true if iterator is empty.
+    pub fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
 }
 
 impl<'s> Iterator for TokenIter<'s> {