Fix up set bindings to handle plus() graphs properly.

Fixes #328

Specifically, Makes plus() resolve parent links, and adds those bindings into the initial binding map added-to by new modules.  SetBindings are special-cased and cloned-in, so new contributors can be added without affecting the parent graph.  SetBindings are also special-cased in the overrides map, and Dagger now throws if one attempts to add set bindings via overrides. 

As a side-effect of the cloning of dependency maps, Linker now needs no parent-linker, since all the bindings are available to a given graph's linker.  So... graphs have a chain of parents, linkers do not. Simpler.