Add `#[must_use]` marker for Buffer

src/lib.rs

@@ -91,6 +91,7 @@ pub struct SamplesRate(pub u32);
 ///
 /// You should destroy this object as soon as possible. Data is only committed when it
 /// is destroyed.
+#[must_use]
 pub struct Buffer<'a, T> {
     // also contains something, taken by `Drop`
     target: Option<cpal_impl::Buffer<'a, T>>,