Push the description of a string stack frame starting at the
top of the string stack and containing u entries onto the
string frame stack. Errors are thrown if the frame stack
would overflow or if the depth of the string stack above the
top frame, if there is one, is less than u. The value u = 0
is allowed. "string-frame"
NOTE: The current implementation pushes u and the string stack pointer
onto the frame stack.
dstrings ordinary primitive