Added String=>Memory and Memory=>String gates. Both gates have a
high-speed interface to easily exchange strings between ZCPU and
non high-speed devices.
Memory => String gate doesn't have a memory limit anymore. Is also allows
reading back the written bytes. (Until now read operations always returned
0)
Fixed unsafe string length handling for Memory => String gate
Merge pull request #543 from lSoleyl/master

Added string-memory conversion gates, which let you access the characters of a string from a hi-speed device and build strings character-by-character.