comb: bug: missing combinations.

The comb function is broken; some combinations of items
are missing in the output. This is because the iteration
reset step in comb_gen_fun_common handles only one
column of the state, neglecting to reset the other
columns: what is now done by the for (j = i ...
loop.  I'm changing the representation of the state from
a list of lists to a vector of lists. Moreover, it is not
reversed. This allows the loop in comb_gen_fun_common
to perform random access.

* combi.c (k_conses): Return a vector, that is not
reversed.
(comb_init): New helper function to slightly abstract
the use of k_conses.
(comb_while_fun): Termination now occurs if the state
vector is nil (degenerate case, like k items chosen
from n, when k > n), or if the vector has nil in
element zero (special flag situation).
(comb_gen_fun_common): Rewritten, with correction.
The logic is similar. Since we have random access,
we don't need the "prev" variable. When we reset
a column iterator, we now also populate all the
columns to the right of it. For instance, if a
given column resets to (a b c), the one to the right
must reset to (b c), and so on. In the broken function,
this is what was not done, resulting in missing
items due to, say, a column resetting to (a b c)
but the one next to it remaining at (c).
(comb_list_gen_fun): Drop nreverse.
(comb_vec_gen_fun, comb_str_gen_fun, comb_hash_gen_fun):
Use the same i iterator for the state and the output object,
accessing the vector directly.
(comb_list, comb_vec, comb_str, comb_hash): Use comb_init.

* tests/015/comb.tl: New file.

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