mapcar: avoid alloca proportional to number of args.

* eval.c (MAP_ALLOCA_LIMIT): New preprocessor symbol.
(map_common): If the number of args is greater than
MAP_ALLOCA_LIMIT, then allocate the array of seq_iter_t
structures from chk_malloc rather than alloca.
In case an exception might be thrown during the execution
of this function, we bind that memory to a buf object.
If we return normally, we call the new function buf_free
to release it. Otherwise we rely on the garbage collector.

* buf.[ch] (buf_free): New function.

* tests/012/seq.tl: Test case which hits this behavior.

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