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_literal_component.cpp

@@ -45,6 +45,8 @@
 #include "util/graph_range.h"
 #include "util/ue2string.h"
 
+#include <unordered_set>
+
 using namespace std;
 
 namespace ue2 {
@@ -196,7 +198,7 @@ bool splitOffLiterals(NG &ng, NGHolder &g) {
     bool changed = false;
     set<NFAVertex> dead;
 
-    ue2::unordered_set<NFAVertex> unanchored; // for faster lookup.
+    unordered_set<NFAVertex> unanchored; // for faster lookup.
     insert(&unanchored, adjacent_vertices(g.startDs, g));
 
     // Anchored literals.