Add an interface to iterate over symbol tables.

Patch originally by Chris Ryder.

Author: martin

src/util/symbol_table_base.cpp

@@ -60,6 +60,16 @@ void symbol_table_baset::show(std::ostream &out) const
     out << symbols.at(name);
 }
 
+symbol_table_baset::const_iteratort symbol_table_baset::end() const
+{
+  return symbols.end();
+}
+
+symbol_table_baset::const_iteratort symbol_table_baset::begin() const
+{
+  return symbols.begin();
+}
+
 /// Print the contents of the symbol table.
 /// \param out: The ostream to direct output to
 /// \param symbol_table: The symbol table to print out

src/util/symbol_table_base.h

@@ -234,6 +234,11 @@ class symbol_table_baset
 
   virtual iteratort begin() = 0;
   virtual iteratort end() = 0;
+
+  using const_iteratort = symbolst::const_iterator;
+
+  const_iteratort begin() const;
+  const_iteratort end() const;
 };
 
 std::ostream &