added support for multiple generics

[siennathesane]

Added support & tests for multiple generics. This allows for complex type support like this:

#[derive(Serialize, Deserialize, ToRedisArgs, FromRedisValue]
pub struct Core<N, E>
where
    N: Node,
    E: EdgeBase,
{
    nodes: BTreeMap<Uuid, N>,
    outgoing_edges: BTreeMap<Uuid, Vec<Uuid>>,
    incoming_edges: BTreeMap<Uuid, Vec<Uuid>>,
    edges: BTreeMap<Uuid, E>,
}

closes #55

[daniel7grant]

Hey Sienna, thank you for contributing and I apologise for not being able to look at it earlier. I didn't find any issue with your implementation, so I'll merge this right away. My only issue is I'd rather separate the multiple generic tests into their own file, but I'll just do this on the master branch if you don't mind.

Thanks for your work!

[daniel7grant]

I published a new version v0.5.6, I hope it works perfectly for you.

[siennathesane]

Sweet, thank you! And yeah, feel free to do whatever you want with the tests, I figured providing them was the important part :)