Update to 1.31

Upstream changes:
1.31 -- Wed Aug 14 20:38 UTC 2013

  * Bugfix pairmap to return list length in scalar context
  * Added "Odd number of elements" warnings to all pair* functions

1.30 -- Mon Aug 05 13:09 UTC 2013

  * Added pairfirst
  * Added MULTICALL implementations to pairmap/pairgrep/pairfirst
  * Fix declaration-after-code for C99-challenged compilers
  * Documentation updates to List::Util

1.29 -- Thu Aug 01 13:40 UTC 2013

  * Bugfix to pairmap/pairgrep when stack moves beneath them during operation

1.28 -- Thu Aug 01 12:19 UTC 2013
  -- BROKEN; do not use. See 1.29

  * Added pairgrep, pairmap, pairs (inspired by List::Pairwise)
  * Added pairkeys and pairvalues

1.26_001 -- Sun Dec 23 15:58

  * Fix multicall refcount bug RT#80646

1.26 -- Sun Dec 16 19:39

  * Merge patch from JDHEDDEN - Add Scalar::Util::isdual() RT#76150

1.25_01 -- Wed Nov 21 09:47

  * Fix a hash order dependency bug t/tainted.t
    (Currently this is a core only version to fix perl5 smokes)

(wen)