Support building against nng as well as nanomsg

[nullr0ute]

With nng now stable and being encouraged as a protocol/wire compatible with nanomsg it would be useful to be able to build nnpy against nng as well as nanomsg.

[djc]

I wouldn't be opposed to having it, but I'm not sufficiently motivated to do it on my own. So, if you want to contribute a PR, I'd be happy to review it!

Others who were asking about nng support thought that the actual nng API (as opposed to the compatibility API) would be too different to support well in nnpy.

[nullr0ute]

I think the compat API would be a useful start as it would allow people to migrate without a lot of code change

[ilyaevseev]

I think the compat API would be a useful start as it would allow people to migrate without a lot of code change

nnpy uses nn_symbol call that is not provided by nng_compat:

• Runtime symbol information is not implemented.
  Specifically, there is no nn_symbol() function yet.
  (This may be addressed later if there is a need.)

[djc]

Someone could probably find the constants some other way (for example, by parsing headers).

[ilyaevseev]

Here is my attempt to create quick&dirty python wrapper for libnng:
https://github.com/ilyaevseev/nngpy/