Infrastructure for calling nodes as lambdas with unique arguments

[Keavon]

Because a Graphene document is turned, at graph compilation time, into a static call graph where each input to a node (its construction argument) is hardcoded in the node's struct as the result of evaluating its input node with static inputs, we can't easily make runtime calls to other nodes with varying inputs. Maybe something equivalent to dynamic dispatch can be employed to solve this limitation, but this will require some discussions with @TrueDoctor about Graphene architecture. The approach taken involves generalizing () and Footprint call arguments with Contexts which allows attaching custom key-value pair data to the call argument when given to predecessor nodes. This can then support varying that data between each repeated call to a predecessor node, like giving a unique index: usize to each iteration's call of a predecessor node.