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/nfa/accel_dfa_build_strat.cpp

@@ -41,6 +41,7 @@
 #include "util/verify_types.h"
 
 #include <sstream>
+#include <unordered_set>
 #include <vector>
 
 #define PATHS_LIMIT 500
@@ -254,7 +255,7 @@ dstate_id_t get_sds_or_proxy(const raw_dfa &raw) {
 
     u16 top_remap = raw.alpha_remap[TOP];
 
-    ue2::unordered_set<dstate_id_t> seen;
+    std::unordered_set<dstate_id_t> seen;
     while (true) {
         seen.insert(s);
         DEBUG_PRINTF("basis %hu\n", s);