added `mean=` aggregator.

added `mean=` aggregator.

`set=` and `bag=` aggregators return dyna lists. So we can use the results in
other rules.

make sure we show changes when we `retract_rule`

when we show changes we print a header

    > a += 1.

    Changes
    =======
    a = 1.

Also, pycall translates python and dyna lists.

Fixed formatting of lists, strings are double quoted.

Fixed `query [A|_] is [1,2,3]` -- we weren't substituting `A`.