std::hash<Guid> implementation for use with std containers

[antiprosynthesis]

Here's a little piece of code that can be added to be able to use Guid as key for containers like std::unordered_map:

namespace std
{
template<>
struct hash<Guid>
{
    size_t operator()(const Guid &guid) const
    {
        stringstream ss;
        ss << guid;
        return hash<string>()(ss.str());
    }
};
}

[fish2000]

Oh wow I just submitted PR #6, with similar implementation (I also expanded stringification, to sidestep the need for the relatively heavyweight std::stringstream invocation). Great minds, yes!

[Justinzobel]

@fish2000 @antiprosynthesis I'd suggest getting together and forking this as it seems abandoned. If you do please tag me here :)

[eliaskosunen]

@Justinzobel I maintain a fork of this repo that implements this and many other™ features: https://github.com/eliaskosunen/crossguid

[graeme-hill]

I just merged it and made a few tweaks to make it compile in android.