feat: Allow clone on backoff builder (#30)

Signed-off-by: Xuanwo <github@xuanwo.io>

src/backoff.rs

@@ -2,7 +2,7 @@ use std::fmt::Debug;
 use std::time::Duration;
 
 /// BackoffBuilder is used to build a new backoff.
-pub trait BackoffBuilder: Default + Debug + Send + Sync + Unpin {
+pub trait BackoffBuilder: Clone + Debug + Send + Sync + Unpin {
     /// The associated backoff that returned by this builder.
     type Backoff: Backoff;
 

src/constant.rs

@@ -31,7 +31,7 @@ use crate::backoff::BackoffBuilder;
 ///     Ok(())
 /// }
 /// ```
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 pub struct ConstantBuilder {
     delay: Duration,
     max_times: Option<usize>,

src/exponential.rs

@@ -36,7 +36,7 @@ use crate::backoff::BackoffBuilder;
 ///     Ok(())
 /// }
 /// ```
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 pub struct ExponentialBuilder {
     jitter: bool,
     factor: f32,