Remove `T: Default` bound from Default impl

Fixes #7

src/lib.rs

@@ -66,7 +66,6 @@ use std::sync::mpsc;
 use std::thread;
 
 /// A builder that runs closures in parallel.
-#[derive(Default)]
 #[must_use]
 pub struct Parallel<'a, T> {
     /// Closures to run.
@@ -263,6 +262,12 @@ impl<T> fmt::Debug for Parallel<'_, T> {
     }
 }
 
+impl<T> Default for Parallel<'_, T> {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 /// Aborts the process if dropped while panicking.
 struct NoPanic;