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/fdr/teddy_compile.cpp

@@ -49,6 +49,7 @@
 #include "util/make_unique.h"
 #include "util/noncopyable.h"
 #include "util/popcount.h"
+#include "util/small_vector.h"
 #include "util/target_info.h"
 #include "util/verify_types.h"