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/nfagraph/ng_prune.cpp

@@ -223,7 +223,7 @@ void pruneHighlanderAccepts(NGHolder &g, const ReportManager &rm) {
 
 static
 bool isDominatedByReporter(const NGHolder &g,
-                           const ue2::unordered_map<NFAVertex, NFAVertex> &dom,
+                           const unordered_map<NFAVertex, NFAVertex> &dom,
                            NFAVertex v, ReportID report_id) {
     for (auto it = dom.find(v); it != end(dom); it = dom.find(v)) {
         NFAVertex u = it->second;