Public ID hash API

[gdamore]

The nni_idhash API is used internally and is super useful there. In the past I've been asked about exposing this, and been hesitant to do so, as I really didn't want to see NNG become a general utility library.

However, more and more people are looking to it for just such a need -- using it to solve their portability problems.

It seems to me that this API is sufficiently useful that it could be exposed under a supplemental API, like I have done for other supplemental APIs.

One thing we might want to look at is whether to also provide a 64-bit ID based variant, as that has been requested in some cases as well. (We used to have one, but I removed it as NNG itself only uses 32-bit IDs at the core. Extending to support 64-bit IDs is relatively straightforward.)