Log Message:
Update to 1.18

Upstream changes:
version 1.18; 2013-09-21

  * bugfix: store peep chain link reliably under threads

  * update tests for Perl 5.19.4's removal of the special treatment of
    aliased undef in arrays

  * in doc, switch to consistent use of British English spellings

  * doc typo fix

version 1.17; 2013-08-04

  * bugfix: correct the dtrace-related macro squashing for Perls prior
    to 5.13.8, where the macros had the wrong number of parameters and
    prevented compilation

  * bugfix: use core's PL_no_localize_ref string constant as data string
    rather than format string

  * update to handle the new padrange op type in Perl 5.17.6

  * update to handle the new op allocation mechanism in Perl 5.17.2

  * clarify and expand documentation around list-like assignments

  * clarify documentation of deref() regarding lvalueness

  * remove the bulk of the documentation from the README file