Add keys, values, and items methods to FermionOperator

[kevinsung]

FermionOperator acts as a dictionary mapping keys to values. It should have keys, values, and items methods.

[kevinsung]

I would like this test to pass:

def test_mapping_methods():
    op = FermionOperator(
        {
            (ffsim.cre_a(1), ffsim.des_a(2)): 1,
            (ffsim.cre_a(2), ffsim.des_a(1)): 0.5,
            (ffsim.cre_b(1), ffsim.des_b(2)): -0.5j,
            (ffsim.cre_b(2), ffsim.des_b(1)): 1 - 0.5j,
        }
    )
    assert op.keys() == {
        (ffsim.cre_a(1), ffsim.des_a(2)),
        (ffsim.cre_a(2), ffsim.des_a(1)),
        (ffsim.cre_b(1), ffsim.des_b(2)),
        (ffsim.cre_b(2), ffsim.des_b(1)),
    }

It turns out to be more complicated than I expected, since the keys method would need to return a Rust object that, for example, knows how to compare correctly to Python objects.

If PyO3/pyo3#991 is implemented, then we could simply inherit from the Python Mapping abstract base class.

[kevinsung]

#119 implements this by adding Python methods to the class dynamically.