ENHANCED: partial_string/3 no longer creates atoms

ENHANCED: partial_string/3 no longer creates atoms

As a consequence, resulting strings are now quickly reclaimed on
backtracking.

This addresses #2912.

Test case:

    :- use_module(library(iso_ext)).
    :- use_module(library(lists)).

    ab(a).
    ab(b).

Sample query:

    ?- length(Ls, 1_000_000),
       maplist(ab, Ls),
       partial_string(Ls, Es0, []),
       Es0 == Ls.
       Ls = "aaaaaaaaaaaaaaaaaaa ...", Es0 = "aaaaaaaaaaaaaaaaaaa ..."
    ;  Ls = "aaaaaaaaaaaaaaaaaaa ...", Es0 = "aaaaaaaaaaaaaaaaaaa ..."
    ;  Ls = "aaaaaaaaaaaaaaaaaaa ...", Es0 = "aaaaaaaaaaaaaaaaaaa ..."
    ;  Ls = "aaaaaaaaaaaaaaaaaaa ...", Es0 = "aaaaaaaaaaaaaaaaaaa ..."
    ;  Ls = "aaaaaaaaaaaaaaaaaaa ...", Es0 = "aaaaaaaaaaaaaaaaaaa ..."
    ;  ... .

running in constant memory.