Define a function called zip that takes an argument of type (Vec A n) and a
second argument of type (Vec B n) and evaluates to a vlue of type (Vec (Pair A B) n),
the result of zipping the first and second arguments.
Define a function called append that takes an argument of type (Vec E m) and an
argument of type (Vect E n) and evaluates to a value of type (Vec (+ m n)), the
result of appending the elements of the second argument to the end of the first.
Define a function called drop-last-k that takes an argument of type (Vec E ?) and
evaluates to a value of type (Vec E ?), the result of dropping the last k elements
from the first argument.
NB: The type of the function should guarantee that we can't drop more elements
than there are in the first argument.