Make union_find use hashes for numbering

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

The type of hash to use is included as a template parameter in order to
make the data structure used to implement `union_find` more explicit.

Author: thomasspriggs

src/solvers/flattening/arrays.h

@@ -68,7 +68,7 @@ class arrayst:public equalityt
   array_equalitiest array_equalities;
 
   // this is used to find the clusters of arrays being compared
-  union_find<exprt> arrays;
+  union_find<exprt, irep_hash> arrays;
 
   // this tracks the array indicies for each array
   typedef std::set<exprt> index_sett;

src/util/union_find.h

@@ -132,11 +132,13 @@ class unsigned_union_find
   size_type get_other(size_type a);
 };
 
-template <typename T>
+/// \tparam T: The type of values stored.
+/// \tparam hasht: The type of hash used for looking up the value numbering.
+template <typename T, typename hasht = std::hash<T>>
 // NOLINTNEXTLINE(readability/identifiers)
 class union_find final
 {
-  using numbering_typet = numbering<T>;
+  using numbering_typet = numberingt<T, hasht>;
   numbering_typet numbers;
 
   // NOLINTNEXTLINE(readability/identifiers)