notecache: add initial in-memory notecache

This is useful for things like relative time strings and other
transient note cache state

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

src/notecache.rs

@@ -0,0 +1,17 @@
+use crate::time::time_ago_since;
+use crate::timecache::TimeCached;
+use std::time::Duration;
+
+pub struct NoteCache {
+    reltime: TimeCached<String>,
+}
+
+impl NoteCache {
+    pub fn new(created_at: u64) -> Self {
+        let reltime = TimeCached::new(
+            Duration::from_secs(1),
+            Box::new(move || time_ago_since(created_at)),
+        );
+        NoteCache { reltime }
+    }
+}