ENHANCED: use a fast test for the expected case of chars in atom_chars/2 etc.

ENHANCED: use a fast test for the expected case of chars in atom_chars/2 etc.

Suggested by Oleg Finkelstein, thank you a lot!

Example, before this change:

    ?- t+\(length(As, 1_000_000), maplist(=(a), As), time(atom_chars(A, As))).
       % CPU time: 0.693s, 7_000_041 inferences
       true.

Now:

    ?- t+\(length(As, 1_000_000), maplist(=(a), As), time(atom_chars(A, As))).
       % CPU time: 0.080s, 40 inferences
       true.

This also partially ameliorates #2907.