CP013: Add way to retrieving current execution resource/context

[AerialMantis]

There was feedback from Jacksonville suggesting that we add a way to retrieve the execution resource or execution context of the current thread of execution.

[hcedwar]

In P0737 there was

  extern const thread_execution_resource_t program_thread_execution_resource ;
  std::this_thread { 
    const thread_execution_resource_t & get_resource();
  }

For the global and local execution resource in which a std::thread could execute.

[AerialMantis]

Resolved by #44.