Support hypermedia links

[Ogeon]

The server should be able to retrieve hypermedia links from its router and deliver them to the handler, as a step towards RESTfulness. The plan is to change the return value from the Router trait to include hypermedia links in some way and add a field for them in Context. It's then up to the handler to send those links in an appropriate format to the client.

TreeRouter should ideally be modified to provide links, based on node children. The problem comes with wildcards and nodes without handlers, so a separate RestRouter with some kind of "default handler" and without support for wildcards would be an alternative.