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Also, fix bug when N is a positive bignum, a problem reported
by Eli Zaretskii and Pip Cet in:
https://lists.gnu.org/r/emacs-devel/2018-08/msg00690.html
* src/fns.c (Fnthcdr): If a cycle is found, reduce the count
modulo the cycle length before continuing.  This reduces the
worst-case cost of (nthcdr N L) from N to min(N, C) where C is
the number of distinct cdrs of L.  Reducing modulo the cycle
length also allows us to do arithmetic with machine words
instead of with GMP.
* test/src/fns-tests.el (test-nthcdr-circular): New test.