Improve debug information from RunnerState

[luketpeterson]

From here: #469 (comment)

@noskill wrote:

I want to access intermediate steps from python to display execution trace like in swi-prolog, though i will have to make additional PRs since there are too many RunnerState and i have hard time finding important steps of interest e.g. backtracking

memb(X, [X| L]).
memb(X, [_| L]) :-
    memb(X, L).

trace:

[trace] 1 ?- memb(A, [1,2]), memb(A, [2,3]).
   Call: (11) memb(_5772, [1, 2]) ? creep
   Exit: (11) memb(1, [1, 2]) ? creep
   Call: (11) memb(1, [2, 3]) ? creep
   Call: (12) memb(1, [3]) ? creep
   Call: (13) memb(1, []) ? creep
   Fail: (13) memb(1, []) ? creep
   Fail: (12) memb(1, [3]) ? creep
   Fail: (11) memb(1, [2, 3]) ? creep
   Redo: (11) memb(_5772, [1, 2]) ? creep
   Call: (12) memb(_5772, [2]) ? creep
   Exit: (12) memb(2, [2]) ? creep
   Exit: (11) memb(2, [1, 2]) ? creep
   Call: (11) memb(2, [2, 3]) ? creep
   Exit: (11) memb(2, [2, 3]) ? creep
A = 2 ;
   Redo: (11) memb(2, [2, 3]) ? creep
   Call: (12) memb(2, [3]) ? creep
   Call: (13) memb(2, []) ? creep
   Fail: (13) memb(2, []) ? creep
   Fail: (12) memb(2, [3]) ? creep
   Fail: (11) memb(2, [2, 3]) ? creep
   Redo: (12) memb(_5772, [2]) ? creep
   Call: (13) memb(_5772, []) ? creep
   Fail: (13) memb(_5772, []) ? creep
   Fail: (12) memb(_5772, [2]) ? creep
   Fail: (11) memb(_5772, [1, 2]) ? creep
false.

It's not perfect representation, but it easier to read than RunnerState, for example

Call: (11) memb(_5772, [1, 2]) ? creep
Exit: (11) memb(1, [1, 2]) ? creep

call here is the same as in metta, it will try to unify variable, Exit means successful unification _5772=1
then the inference engine tries memb(A=1, [2,3]) and fails
"Redo: (11) memb(_5772, [1, 2]) ? creep" is backtracking to previously tried variant A=1

The same program in metta

        (= (memb $X Nil) False)
        (= (memb $X (Cons $H $Tail))
           (memb $X $Tail))
        (= (memb $X (Cons $X $Tail))
           True)
        !(let $res (and (memb $X (Cons 1 (Cons 2 Nil))) (memb $X (Cons 2 (Cons 3 Nil))))   (if $res $X None))