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.

unit/internal/flat_set.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015, Intel Corporation
+ * Copyright (c) 2015-2017, Intel Corporation
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are met:
@@ -28,7 +28,7 @@
 
 #include "config.h"
 
-#include "util/ue2_containers.h"
+#include "util/flat_containers.h"
 #include "ue2common.h"
 
 #include "gtest/gtest.h"
@@ -393,6 +393,11 @@ TEST(flat_set, max_size) {
     ASSERT_LE(1ULL << 24, f.max_size());
 }
 
+template<typename FlatSet>
+size_t hash_value(const FlatSet &f) {
+    return std::hash<FlatSet>()(f);
+}
+
 TEST(flat_set, hash_value) {
     const vector<u32> input = {0,        15, 3,   1,   20,  32768,
                                24000000, 17, 100, 101, 104, 99999};