Make numbering use unordered_map if numbering ireps

This avoids slow lookups into `std::map` caused by long running
`irept::operator<` calls.

Author: thomasspriggs

src/util/numbering.h

@@ -12,9 +12,11 @@ Author: Daniel Kroening, [email protected]
 #include <map>
 #include <unordered_map>
 #include <vector>
+#include <type_traits>
 
 #include "invariant.h"
 #include "optional.h"
+#include "irep.h"
 
 /// \tparam Map: a map from a key type to some numeric type
 template <typename Map>
@@ -110,7 +112,10 @@ class template_numberingt final
 };
 
 template <typename Key>
-using numbering = template_numberingt<std::map<Key, std::size_t>>; // NOLINT
+using numbering = typename std::conditional<
+  std::is_base_of<irept, Key>::value,
+  template_numberingt<std::unordered_map<Key, std::size_t, irep_hash>>,
+  template_numberingt<std::map<Key, std::size_t>>>::type;
 
 template <typename Key, typename Hash>
 using hash_numbering = // NOLINT