Leave the string stack unchanged, and leave the string body address and
length on the data stack. "string-s-fetch"
NOTE: In earlier versions this was call $S@S. The trailing "S" is
superfluous if it is understood that the only string format that
usually appears on the data stack is the Forth string format.
WARNING: If a$ is a dynamic string, it may move at the next garbage
collection, making a.str invalid.
dstrings ordinary primitive