Handle unicode text encoding and text escape sequences

* Fix echo() encoding of strings (#838)
* Add escape sequence handling \x \u and \U

ToDo:
* [ ] Validate if https://github.com/openscad/openscad/compare/text-escape-sequences#diff-efe9f8f7d601fffd8bedc312e8713866R202 works on different endian
* [ ] Add fix for #737