This commit should reduce the amount of iterations in concatenate to N
where N is the sum of the lengths of the input lists.
The previous implementation iterated from the beginning in each
concatenation because of `last-pair`.
This implementation is significantly faster in this extreme case:
(concatenate! `(,(iota 50000) ,@(map list (iota 500))))
>> Previous implementation:
real 0m0.671s
user 0m0.658s
sys 0m0.013s
>> This implementation:
real 0m0.175s
user 0m0.174s
sys 0m0.001s
The tests is done using `time`, which is not reliable at all, but using
`(trace last-pair)` shows accurately what happens with the iterations.
