Use static typechecking with generics over runtime type checking in Slice interface

[YeungOnion]

In the docs, I saw that Slice mentions how Go doesn't have generics and therefore relies on dynamic typechecking. Well now we have them!

Could potentially solve #115 by failing compilation - the earliest failure.

Would it be worth the squeeze to replace the dynamic typechecking with static via generics?

If it seems worth a short for a good first issue, I'd work on this.