Rename Iop_Extract{64,V128} to Iop_Slice{64,V128}, improve their
documentation, and swap the sense of the first and second args
so as to be more in keeping with the rest of the ops here, so
that the more significant arg is arg1 rather than arg2.



git-svn-id: svn://svn.valgrind.org/vex/trunk@2939 8f6e269a-dfd6-0310-a8e1-e2731360e62c