New feature: range iteration with skip.

The notation X..Y..Z now denotes an iterable range,
if X..Y is a valid iterable range on its own, and Z is a
positive integer. Z gives a step size: 1 takes every
element, 2 every other and so on.

* lib.c (seq_iter_get_skip, set_iter_peek_skip): New static
functions.
(si_skip_ops): New static structure.
(iter_dynamic): Function relocated earlier in file to avoid
forward declaration.
(seq_iter_init_with_info): When the iterated object is a
range, check for the to element itself being a range.
If so, it is potentially a skip iteration. Validate it
and implement via a skip iterator referencing a dynamic
range iterator.

* lib.h (struct seq_iter): New sub-union member, ul.skip.
We could use an existing member of type cnum;
this is for naming clarity.

* tests/012/iter.tl: New tests.

* txr.1: Documented.

1 files changed, 1 insertions, 0 deletions

diff --git a/lib.h b/lib.h
index 649fc291..2d2b5e4e 100644
--- a/lib.h
+++ b/lib.h
@@ -454,6 +454,7 @@ typedef struct seq_iter {
     cnum len;
     val vbound;
     cnum cbound;
+    cnum skip;
     val next;
     val dargs;
   } ul;