inserto a l p: p is l with a inserted at some position. Recursive: head
of l first, then push past head and recurse.
permuteo l p: classical recursive permutation. Empty -> empty; otherwise
take a head off l, recursively permute the tail, insert head at any
position in the recursive result.
7 new tests including all-6-perms-of-3 as a set check (independent of
generation order). 208/208 cumulative.