Make the symbol_table_baset define the const iterators

Since the const iterators can use the base class directly, these are not
pure virtual - so derived classes can still provide an entirely
different mechanism for iterating over the symbols - though this is not
currently used.

Author: Thomas Kiley

src/util/journalling_symbol_table.h

@@ -140,6 +140,9 @@ class journalling_symbol_tablet : public symbol_table_baset
       base_symbol_table.end(), [this](const irep_idt &id) { on_update(id); });
   }
 
+  using symbol_table_baset::begin;
+  using symbol_table_baset::end;
+
   const changesett &get_inserted() const
   {
     return inserted;

src/util/symbol_table.h

@@ -120,17 +120,8 @@ class symbol_tablet : public symbol_table_baset
     return iteratort(internal_symbols.end());
   }
 
-  typedef symbolst::const_iterator const_iteratort;
-
-  virtual const_iteratort begin() const
-  {
-    return internal_symbols.begin();
-  }
-
-  virtual const_iteratort end() const
-  {
-    return internal_symbols.end();
-  }
+  using symbol_table_baset::begin;
+  using symbol_table_baset::end;
 
   /// Check that the symbol table is well-formed
   void validate(const validation_modet vm = validation_modet::INVARIANT) const;

src/util/symbol_table_base.h

@@ -237,8 +237,8 @@ class symbol_table_baset
 
   using const_iteratort = symbolst::const_iterator;
 
-  const_iteratort begin() const;
-  const_iteratort end() const;
+  virtual const_iteratort begin() const;
+  virtual const_iteratort end() const;
 };
 
 std::ostream &

src/util/symbol_table_builder.h

@@ -85,6 +85,9 @@ class symbol_table_buildert : public symbol_table_baset
     return base_symbol_table.end();
   }
 
+  using symbol_table_baset::begin;
+  using symbol_table_baset::end;
+
   /// Try to find the next free identity for the passed-in prefix in
   /// this symbol table.
   /// \remark