Trivial comment modification
Bugfix join-pairs-to-coll-map now guarantees vals are colls

When there was only one value to collect, merge-with does nothing,
so these cases ended up with values that were single elements, rather
than singleton vectors, as intended.

Also added the following functions:

collect
collectivize
map-keys
map-vals
map-keys-vals

map-vals and collectivize were used in the bugfix.
Commented out new function collect, since not using it.