util: switch from Boost to std::unordered set/map

This commit replaces the ue2::unordered_{set,map} types with their STL
versions, with some new hashing utilities in util/hash.h. The new types
ue2_unordered_set<T> and ue2_unordered_map<Key, T> default to using the
ue2_hasher.

The header util/ue2_containers.h has been removed, and the flat_set/map
containers moved to util/flat_containers.h.

src/util/ue2string.cpp

@@ -29,11 +29,14 @@
 /** \file
  * \brief Tools for string manipulation, ue2_literal definition.
  */
-#include "charreach.h"
-#include "compare.h"
+
 #include "ue2string.h"
 
+#include "charreach.h"
+#include "compare.h"
+
 #include <algorithm>
+#include <cstring>
 #include <iomanip>
 #include <sstream>
 #include <string>