kana_transliteration: handle k->h itermark translit, add more tests

src/util.rs

@@ -0,0 +1,21 @@
+pub(crate) fn transpose_codepoints_in_range(
+    text: &str,
+    distance: i32,
+    range_start: u32,
+    range_end: u32,
+) -> String {
+    debug_assert!(range_start <= range_end);
+    debug_assert!(i64::from(range_start) + i64::from(distance) >= 0);
+    debug_assert!(i64::from(range_end) + i64::from(distance) <= i64::from(u32::MAX));
+
+    text.chars()
+        .map(|c| {
+            let codepoint = c as u32;
+            if range_start <= codepoint && codepoint <= range_end {
+                std::char::from_u32(codepoint.strict_add_signed(distance)).unwrap_or(c)
+            } else {
+                c
+            }
+        })
+        .collect()
+}