Make renamedt inherit from underlyingt

[romainbrenguier]

This is so that we can easily convert from an exprt to a renamedt using
static_cast.
The potential use case, is to be able to look at all the operands of a
renamedt, as renamedt themselves without having to construct new
objects.
For instanc, we could declare a renamedt method:

void renamedt::apply_to_operands(std::function<void(renamedt&)> f)
{
  for(exprt &op : operands())
    f(static_cast<renamedt&>(op));
}

The idea for correctness is that if an expression is renamed then all its operands also are, by construction.

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[smowton]

Counter-proposal: smowton@1daa2c6

Already suggested that to @owen-jones-diffblue

[allredj]

⚠️
This PR failed Diffblue compatibility checks (cbmc commit: aa3f0f1).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/106290116
Status will be re-evaluated on next push.
Common spurious failures include: the cbmc commit has disappeared in the mean time (e.g. in a force-push); the author is not in the list of contributors (e.g. first-time contributors); compatibility was already broken by an earlier merge.

[owen-mc-diffblue]

I will rebase #4444 on this so we can avoid calling constructors all the time