Add support for HashMaps

[ahqsoftwares]

Stabby is great, though actually I think having a HashMap implementation would be a great addition to this library. It would allow us to safely share a HashMap b/w dynamic libraries.

[p-avital]

Hi there,

Sorry for the high latency, I just realized that I wasn't actually subscribed to issues on the repo anymore.
The difficult part with HashMap is that they're a lot more complicated than most types to implement properly.

I'd suggest creating a Map trait and passing that around instead. I might take a stab at this later, but I first want to address the quite large set of issues that's accumulated over my time un-notified :)