create a new offsetword. The word is created and upon execution
it adds the offset, ie. compiling the OFFSET-RT runtime:
( address -- address+offset )
This word is just a convenience word, just use the word +FIELD directly and choose a DROP to flag the end of a current
offset-field declaration series. See also /FIELD series to
declare simple structures which end with a final CONSTANT to
memorize the complete size. The /FIELD style is more traditional.