Add back repr for Result and related classes

[mtreinish]

Summary

In the removal of marshmallow from the Result class (and terra as a
whole) in #4030 the descriptive repr for the objects was lost. This is
inconvenient because the default representation for an object is just
the class name and the memory address of the object. This commit adds
back a string representation of the objects which were missing by adding
a repr method to the classes.

Details and comments

[mtreinish]

If people are curious an example output from repr(Result) is:

Result(backend_name='2.0.0', backend_version='2.0.0', qobj_id='d0d7793e-743a-4af1-ab6a-c635c42e3c95', job_id='2e80dbb7-e368-47fa-b96e-217363823d2f', success=True, results=[ExperimentResult(shots=1024, success=True, meas_level=MeasLevel.CLASSIFIED, data=ExperimentResultData(counts={'0x0': 525, '0x3': 499}), header=QobjExperimentHeader(clbit_labels=[['c', 0], ['c', 1]], creg_sizes=[['c', 2]], global_phase=0, memory_slots=2, n_qubits=2, name='circuit0', qreg_sizes=[['q', 2]], qubit_labels=[['q', 0], ['q', 1]]), status=DONE, name='circuit0', seed_simulator=670299412, time_taken=0.0029897689819335938)], status=COMPLETED, status=QobjHeader(), time_taken=0.0030059814453125)

[1ucian0]

Does it make sense to add a test for this?

[mtreinish]

I can, but it's not like it's a critical piece it's just a string representation of the data in there.