Merge pull request #137 from labster/nom

fix jnthn++'s report of infinite recursion in IO::Spec::Unix.rel2abs
constant-fold infix:<~>
potentially constant-fold more stringy operators
potentially constant-fold &defined
Add default method for Set, KeySet, Bag, and KeyBag.
[t/spectest.data] chase rename/addition of test files

For lizmat, as per http://irclog.perlgeek.de/perl6/2013-05-15#i_7070799