Remove MappedNode & generalise render logic

[tomwilkie]

Towards #228

[peterbourgon]

Could you write a few sentences for context? At a minimum, the problem that motivates this PR, and a high-level description of the implementation?

[tomwilkie]

This is a refactoring on the path to synthesising the containers topology from endpoints topology via the process topology. To do this we need the renderers to be able to recurse - effectively what I'm aiming for is:

render.Reduce{
    render.LeafMap{Selector: report.SelectContainer, Mapper: report.MapContainerIdentity, Pseudo: report.InternetOnlyPseudoNode},
    render.Map{
        Mapper: report.MapProcess2Container,
        Renderer: render.Reduce{
            render.LeafMap{Selector: report.SelectProcess, Mapper: report.MapProcessIdentity, Pseudo: report.InternetOnlyPseudoNode},
            render.Map{
                Mapper: report.MapEndpoint2Process,
                Renderer: render.LeafMap{Selector: report.SelectEndpoint, Mapper: report.MapEndpointIdentity, Pseudo: report.InternetOnlyPseudoNode},
            }
        }
    }
}

This isn't all the way there yet, just a step in that direction (getting the Renderer interface right I think).

I removed MappedNode as its redundant, with the introduction of DetailedNode.Merge. It has the nice side effect of propagating Origin nodes correctly for nodes without any adjacency info.

[peterbourgon]

This feels really refreshing. Whenever we can eliminate a needless intermediate form, wonderful. Thanks! Those comments addressed, LGTM.