feat: use unordered_map instead of map

lib_vector_search/include/bucket_finder.h

@@ -3,11 +3,11 @@
 #include "finder.h"
 #include "word_list.h"
 
-#include <map>
+#include <unordered_map>
 
 class Bucket {
 private:
-  std::map<char, WordRefList> directory_;
+  std::unordered_map<char, WordRefList> directory_;
 
 public:
   void insert(const WordList &word_list, size_t first_index, size_t last_index);

lib_vector_search/include/grouped_finder.h

@@ -3,11 +3,11 @@
 #include "finder.h"
 #include "word_list.h"
 
-#include <map>
+#include <unordered_map>
 
 class GroupedFinder : public Finder {
 private:
-  std::map<char, WordRefList> groups_;
+  std::unordered_map<char, WordRefList> groups_;
 
 public:
   GroupedFinder(const WordList &word_list);

lib_vector_search/include/tree_finder.h

@@ -3,12 +3,12 @@
 #include "finder.h"
 #include "word_list.h"
 
-#include <map>
+#include <unordered_map>
 
 class SearchTreeNode {
 private:
   WordRefList words_;
-  std::map<const char, SearchTreeNode> children_;
+  std::unordered_map<char, SearchTreeNode> children_;
 
 public:
   void insert(std::string_view partial_word, const std::string *original_word);

lib_vector_search/src/grouped_finder.cpp

@@ -1,6 +1,5 @@
 #include "grouped_finder.h"
 
-#include <forward_list>
 #include <mutex>
 #include <thread>
 #include <vector>