Refactor `xcol`

[codereport]

In jsrc/verbs/monadic/tally.cpp:

[[nodiscard]] auto
jttally(J jt, array w) -> array {
    array const z = make_scalar_integer(jt, item_count(w));
    return w->tproxy.t & extended_precision_or_rational() ? xco1(z) : z;  // TODO: xcol
}

[juntuu]

The example is outdated, also xco1 should go away as xnum support is removed, so this issue might not be relevant anymore.