Fixes #271 (closed).

This big MR refactors the whole API to use Servant's named routes which offers a lot of advantages over plain routes, including:

• A better (i.e. shorter and more precise) error messages in case we forget to handle some routes; no more errors spanning thousands of lines of terminal screen;
• A better way to generate and use clients. This is the client for the full TableNgramsApiPut, for example:

table_ngrams_put_api :: Token
                     -> NodeId
                     -> TabType
                     -> ListId
                     -> Versioned NgramsTablePatch
                     -> ClientM (Versioned NgramsTablePatch)
table_ngrams_put_api (toServantToken -> token) nodeId =
  clientRoutes & apiWithCustomErrorScheme
               & ($ GES_new)
               & backendAPI
               & backendAPI'
               & mkBackEndAPI
               & gargAPIVersion
               & gargPrivateAPI
               & mkPrivateAPI
               & ($ token)
               & nodeEp
               & nodeEndpointAPI
               & ($ nodeId)
               & tableNgramsAPI
               & tableNgramsPutAPI
               & putNgramsTableEp

I find this lovely, it's just function application all the way down, and for the handlers that excepts to receive arguments (like the Token to prove we are authenticated, or the given NodeId, we supply those). I think @cgenie and I will really appreciate this, especially when writing tests, which can now move away from using stringly-typed endpoints, that sometimes where necessary to deal with the complexity of generating clients in the first place. Not anymore!

In the future we can integrate the printRoutes facility, but let's do it in another MR, as this one is already incredibly long.

@anoe Sorry for taking so long but I had to learn along the way how to write code using the generic routes; once CI passes, you are free to merge this.