Merge derived LayerApi and HateoasApi

[bruderj15]

Currently deriving the HATEOAS Layer-Api (and server) and the rewritten HATEOAS-Api exist in isolation.

In the example we get:

layerServer :: Server (Resourcify (MkLayers Api) (HAL JSON))
layerServer = getResourceServer (Proxy @Handler) (Proxy @(HAL JSON)) (Proxy @(MkLayers Api))

apiServer :: Server (Resourcify Api (HAL JSON))
apiServer = getResourceServer (Proxy @Handler) (Proxy @(HAL JSON)) (Proxy @Api)

We would want:

layerAndApiServer :: Server (Resourcify (MkLayers Api) (HAL JSON) :<|> Resourcify Api (HAL JSON))
layerAndApiServer = getResourceServer (Proxy @Handler) (Proxy @(HAL JSON)) (Proxy @(MkLayers Api))
               :<|> getResourceServer (Proxy @Handler) (Proxy @(HAL JSON)) (Proxy @Api)

This may lead to multiple server-implementations for the same endpoint.

Try an approach where we merge both server-implementations by adding all links from the one in layerServer to the one in apiServer, then discarding the one in layerServer.