Escape single \ (that is not in a \n)

* escape:
  * first newline is escaped to \n ,
  * now a real \ needs to be escaped to \\
    (otherwise a genuine \n would be unescaped to newline)

* unescape:
  * first finds \n after even number of \\ and unescape to newline
  * then unescape pairs of \\ to single \

* TestFactory::Fact.data_fact_EU has a complex pattern to test this