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.

util/ng_find_matches.cpp

@@ -41,6 +41,7 @@
 #include "util/compare.h"
 #include "util/report.h"
 #include "util/report_manager.h"
+#include "util/unordered.h"
 
 #include <algorithm>
 
@@ -770,7 +771,7 @@ struct EdgeCache {
     }
 
 private:
-    unordered_map<pair<NFAVertex, NFAVertex>, NFAEdge> cache;
+    ue2_unordered_map<pair<NFAVertex, NFAVertex>, NFAEdge> cache;
 };
 
 struct fmstate {