Fix Module::display to print function bodies again

[daxhuiberts]

The change which added PrintDecorator broke the default Module::display by not showing the function bodies anymore when no PrintDecorator was provided.

This change always invokes the body display function again, now with the optional PrintDecorator.

This does change the signature of FunctionBody::display_with_decorator to take an Option<&'a PD> now, which is not consistent with the function convention on Module, but this needs a display function on FunctionBody which takes an Option.

Alternatively I can change this to invoke FunctionBody::display or FunctionBody::display_with_decorator depending on the presence/absence of a decorator, but that feels less ideal.

[cfallin]

Thanks very much!

[daxhuiberts]

I also saw that ModuleDisplay and FunctionBodyDisplay are generic over PrintDecorator.
Perhaps they should not be generic, but accept an Option<&'a dyn PD> which feels more appropriate here.
This will also remove the need of NOPPrintDecorator.

I can make a follow-up PR with this change if you're welcome to it.