add basic exchange util

Signed-off-by: Jason Volk <jason@zemos.net>

src/core/utils/mod.rs

@@ -26,8 +26,16 @@ pub use string::{str_from_bytes, string_from_bytes};
 pub use sys::available_parallelism;
 pub use time::now_millis as millis_since_unix_epoch;
 
+#[inline]
 pub fn clamp<T: Ord>(val: T, min: T, max: T) -> T { cmp::min(cmp::max(val, min), max) }
 
+#[inline]
+pub fn exchange<T: Clone>(state: &mut T, source: T) -> T {
+	let ret = state.clone();
+	*state = source;
+	ret
+}
+
 #[must_use]
 pub fn generate_keypair() -> Vec<u8> {
 	let mut value = rand::string(8).as_bytes().to_vec();