Inline computeNextState and computeHandlerProps
Added initialPath and initialMatches to <Routes>

These are currently only used for server-side rendering.
Fix failing tests
Use class set for generating <Link> className
[added] Server-side rendering

This commit adds two functions:

1. Router.renderRoutesToStaticMarkup(routes, path, callback)
2. Router.renderRoutesToString(routes, path, callback)

These methods are the equivalents to React's renderComponentTo*
methods, except they are designed specially to work with <Routes>
components.

This commit obsoletes #181. Many thanks to @karlmikko and others
in that thread for getting the conversation going around how this
should all work.