Pushing a persona multiple times

The following contrived example pushes a persona a second time on the same persona stack:

#include <upcxx/upcxx.hpp>

int main() {
  upcxx::init();
  {
    upcxx::persona_scope s(upcxx::default_persona());
  }
  upcxx::finalize();
  return 0;
}

This is permitted by the wording in the current spec, but fails at runtime in the current implementation, both seq and par backends:

UPC++ assertion failure on rank 0 [/home/pcp1/bonachea/UPC/inst-upcxx/upcxx.debug.gasnet1_seq.smp/include/upcxx/persona.hpp:188]: Persona already in some thread's stack.

The example above is obviously silly, but there may be more complex progress-thread codes that may want to do this (using non-default personas), without having to check if the persona in question is already pushed on the stack (especially since we don't provide a means to query the current stack contents).

We need to either fix the implementation to support this, or change the spec to prohibit it.

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