Improve socket comparison operators (needed for Catch2)

[albestro]

Reporting here the discussion with the aim to evaluate the best option to improve them.

There still is a problem involving socket_ref and comparison operators, that I quickly workaround (not committed). I'll investigate further and in case I will open an issue to understand the best way to fix it.

Ok, I've just narrowed down the problem. For socket_ref a set of comparison operators is defined (starting from here), moreover it is possible to implicitly cast a socket_t to a socket_ref thanks to

cppzmq/zmq.hpp

Line 2186 in 18db456

operator socket_ref() ZMQ_NOTHROW { return socket_ref(from_handle, _handle); }

so, it is possible to use these comparison operators with both types.

But with Catch a problem arose, because CHECK(sr == s) passes by const& the operands and the cast operator for const socket_t is not available.

Unfortunately, I'm not used to the API of the library, so I would have to dig a bit to understand what's better to do (e.g. enabling the socket_ref operator by adding const). I can commit this proposal, just to see if other problems will come up in CI.

I don't know if you want me to open an issue about this, or if we want to keep it here and solve it directly.

Originally posted by @albestro in #466 (comment)

[albestro]

@gummif if I get correctly your suggestion in #466 (comment), the solution would be to implement the different combinations for the various operators.

I was going to quickly implement it, but I'm not really sure that it is the best solution in terms of maintainability.

What do you think about something similar to this? https://github.com/albestro/cppzmq/commit/80914202e44dfbe0b74496333801d8deb2ccc446

[gummif]

@albestro Yes that could would (if you use const &).