This should simplify and improve implementation, make it easier to
implement template strings, and keep master a bit more in sync with
harmony.
Previous implementation wasn't broken, though the loop gave me the
impression it could read infinite numbers and annoyed me a bit. It was
also slightly unnecessary because the lookup involved only 3 characters.
* Don't interpret strings with escaped content as directive
* Don't interpret strings after empty statement as directive
* Adapt output to prevent strings being represent as directive
* Introduce UGLIFY_DEBUG to allow internal testing like EXPECT_DIRECTIVE