Add trait for single unlocking of an item

Add a trait serving a lock which features only an irreversible unlocking
of an item. Such a type may be used to pass around locked items while
permitting the user to unlock them and perform any operation with a copy
of the original item.

libimagstore/src/single_use_lock.rs

@@ -0,0 +1,5 @@
+pub trait SingleUseLock<T> {
+    fn access(&self) -> &T;
+    fn unlock(self) -> T;
+}
+