Add `improper-list?`

[jpellegrini]

Not sure if this reeeealy is interesting, but here it is: it can be useful to make code clearer -- instead of

(and (pair? x) (not (list? x)))

we can write

(improper-list? x)

[egallesio]

Hi @jpellegrini, As you said, not really useful. But why not? :smile: Merged.