Drop "stdlib" Router

[marwan-at-work]

~For both server/kit, and SimpleServer~ For server/kit, is there a reason for having a user be able to override the gorilla/mux Router? The only two reasons I can think of are:

1. Performance reasons: but I imagine if a user cared about the performance delta between stdlib vs mux, then Gizmo is the wrong library to use in the first place.
2. Different features/functionality: but we don't expose those functionalities to the end user anyway.

Unless I'm missing an obvious reason here, we should probs just go with gorilla/mux as the default router.

I know we can also let the user pass their own mux for server/kit, but in that case, they have any mux they wish so it's a bit unrelated to this issue.

Thanks!

[jprobinson]

I'm fine with dropping stdlib and leaning purely on gorilla/mux. I think this just came from the "router wars" days. I shouldn't have accepted those PRs 😅