Add const iterator to symbol table

This adds const iterators to iterate over the internal_symbols map of the symbol
table. So far, only non-const iterators existed.

Author: danpoe

src/util/symbol_table.h

@@ -117,6 +117,18 @@ 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();
+  }
+
   /// Check that the symbol table is well-formed
   void validate(const validation_modet vm = validation_modet::INVARIANT) const;