Derive Copy trait to SizeHint struct

The struct has only a size of 24 bytes and is trivially copy-able.

Author: cgzones

http-body/src/size_hint.rs

@@ -4,7 +4,7 @@
 ///
 /// * 0 for `lower`
 /// * `None` for `upper`.
-#[derive(Debug, Default, Clone)]
+#[derive(Debug, Default, Clone, Copy)]
 pub struct SizeHint {
     lower: u64,
     upper: Option<u64>,