`dynvector-fold`procedureLeft-to-right dynamic vector iterator with state.

`f`is iterated over each index in all of the vectors, stopping at the end of the shortest;`f`is applied as`(f i state (dynvector-ref x1 i) ...)`where state is the current state value, which begins with`initial`and becomes whatever`f`returns at the respective iteration;`i`is the current index.