ALLOT room and store the Forth string into data space as an
mstring, leaving data space aligned; and leave the length and
new body address. It is assumed that len is unsigned. An
error is thrown if len is larger than the system parameter
NOTE: MAX_DATA_STR is returned by
S" /SCOPY" ENVIRONMENT?
Perhaps this restriction should be removed in favor of a
normal data space overflow error.
NOTE: S, is the same as STRING, in Wil Baden's Tool Belt,
except it stores a measured string instead of a counted