Append the Forth string body to the end of the string
currently being concatenated as the last string in the string
buffer, and update its count field. If there is no
concatenating string, start one. An error is thrown if the
size of the combined string would be larger than MAX_MCOUNT
or if there is not enough room in string space even after a
S-CAT is most commonly used on external strings, not assumed
to exist as mstrings. In contrast to CAT, garbage
collection could invalidate a.str if it is a dynamic string
in the string buffer. S-CAT can be used in that situation if
garbage collection is turned off with $GC-OFF.
When there is a concatenating string, concatenation is the
only basic string operation that can copy a string into the
string buffer. "s-cat"