timeline: add ui_abbrev_name

This is a simple and fast function for abbreviating usernames in the
timeline. If they are too large they make the timeline too wide. We'll
probably need to make this adjustable based on the timeline width, but
this is ok for now.

Changelog-Added: Abbreviate usernames in timelines

src/abbrev.rs

@@ -0,0 +1,20 @@
+#[inline]
+pub fn floor_char_boundary(s: &str, index: usize) -> usize {
+    if index >= s.len() {
+        s.len()
+    } else {
+        let lower_bound = index.saturating_sub(3);
+        let new_index = s.as_bytes()[lower_bound..=index]
+            .iter()
+            .rposition(|b| is_utf8_char_boundary(*b));
+
+        // SAFETY: we know that the character boundary will be within four bytes
+        unsafe { lower_bound + new_index.unwrap_unchecked() }
+    }
+}
+
+#[inline]
+fn is_utf8_char_boundary(c: u8) -> bool {
+    // This is bit magic equivalent to: b < 128 || b >= 192
+    (c as i8) >= -0x40
+}