Generalize code in els_methods

[robertoaloi]

Following up on the review of #1329:

I think, though, this can go a step further and remove even more abstraction. For example, rather than having code like:

Provider = els_call_hierarchy_provider,
Request = {incoming_calls, Params},
els_provider:handle_request(Provider, Request)

I think it would be much clearer to see:

els_call_hierarchy_provider:incoming_calls(Params)

And I don't think it would loose any generality compared to the current implementation.