dvec and vec interface cleanup #2924
Description
Vec needs 'reach'. (fixed)
Dvec needs 'reverse'. (fixed)
Dvec 'reach' doesn't... reach. i.e., it iterates in fwd order. (fixed)
many should be pure that aren't (dvec len) (fixed)
dvec needs 'view' (didn't bother)
the fold* suite is really messy when constness doesn't match up - maybe have one for each constness? (didn't bother)
Found during ICFP (#2928).