Simplify Router

[maciejhirsz]

Building upon #95:

• The way it was built there was no reason for putting the router in an Rc that I can see.
• I've removed the Params hashmap and the whole need to pass it over the browser state, instead a newtype wrapper over matchit::Params is being piped into the route functions.
• The Params::get method replaces get_param.
• I've hidden the start_route function in the add_route method and made it an implementation detail (removed pub).
• Removed route_view! macro as add_route just accepts a regular Fn(Params) -> impl View closure.

The startup can also be simplified, but we'll need way to turn arbitrary impl View into a dynamic boxed view type (with a boxed product) first.