Package cxx was added in 9785f2d3 (cxx: New package), but the interface
that cxx:dict provided turned out to be not optimal:

    dict.get  was returning (v, ok), and
    dict.pop  ----//---

Correct dict.get and dict.pop to return just value, and, similarly to
channels API, provide additional dict.get_ and dict.pop_ - extended
versions that also return ok:

    dict.get(k)  -> v
    dict.pop(k)  -> v
    dict.get_(k) -> (v, ok)
    dict.pop_(k) -> (v, ok)

This time add tests.