String configuration for better usability

[ghost]

There seems to be a demand for a more consistent and
bug free handling of strings. At least I myself would be
happy if quotes were thoroughly handle by the corresponding
flags. The behaviour could be such:

?- set_prolog_flag(single_quotes, chars).
Yes
?- set_prolog_flag(double_quotes, atom).
Yes
?- X = "Foo".
X = "Foo"
?- X = 'Foo'.
X = ["F",o,o]