assocs.extras: Fix error with assoc-collapse with f as first element

assocs.extras: Fix error with assoc-collapse with f as first element

Since `assoc-collapse` uses a clone of the first assoc in the input sequence as
basis for the resulting assoc, this was failing when the first element was `f`.
Change behavior to return a hashtable instead, since this is the
default behavior for `f new-assoc` as well.