timecache: add timecache help for timed caches

Some things we definitely don't want to generate every frame, such as
relative-time formatted strings, as that would create a heap allocation
each frame.

Introduce TimeCached<T> which is responsible for updating some state
after some expiry.

Signed-off-by: William Casarin <jb55@jb55.com>

src/lib.rs

@@ -9,6 +9,7 @@ mod images;
 mod result;
 mod filter;
 mod ui;
+mod timecache;
 mod frame_history;
 mod timeline;