Do more lookups, but try to avoid the largests bounds this way

sa-index/src/sa_searcher.rs

@@ -308,9 +308,12 @@ impl Searcher {
         // Use the (up to) first 5 characters of the search string as the kmer
         // If the kmer is found in the cache, use the bounds from the cache as start bounds
         // to find the bounds of the entire string
-        let max_mer = &search_string[..min(5, search_string.len())];
-        if let Some(bounds) = self.kmer_cache.get_kmer(max_mer) {
-            return self.search_bounds_no_cache(search_string, bounds);
+        let mut max_mer_length = min(5, search_string.len());
+        while max_mer_length > 0 {
+            if let Some(bounds) = self.kmer_cache.get_kmer(&search_string[..max_mer_length]) {
+                return self.search_bounds_no_cache(search_string, bounds);
+            }
+            max_mer_length -= 1;
         }
 
         BoundSearchResult::NoMatches