libzmq is a fast-chanding beast and constants get renamed, new one gest
removed, etc...

We used to auto-generate constants from the libzmq source code, but then
adpating the binding code to this change got very tedious, and controling
which version contains which constants got very hard to manage.

This module is now separate from ZMQ main code, and lists the constants
statically. You can also specify which set of constants to pull in
depending on the zmq version.

If don't care to be strict about which constants to import into your code,
then don't bother with the ':vX.X.X' notation, just use like a normal
module:

use ZMQ::Constants qw(:all); # pulls all known constants...
# note: may contain extra constants
# if we have clashes in the future

use ZMQ::Constants qw(ZMQ_PUSH); # import just ZMQ_PUSH