[PSyIR] routine_symbol property

[JulienRemy]

For convenience, I suggest adding a routine_symbol property in Routine to act as a getter of Routine(...).symbol_table.lookup_with_tag("own_routine_symbol"), which is a bit long.

I do not think a setter is needed, as: 1/ it's already dealt with by the name setter, 2/ Routine.create takes a string name and a not a RoutineSymbol instance as argument.

[sergisiso]

I will close this since it is a subset of #2592 functionality which will introduce the routine.symbol