Context to contain `to-ref` function

[den1k]

We're often faced with the unknown of wether a ref is a ref or whether we need to convert it. We've been using helper functions convert whatever is ref-convertible before calling pull. To avoid converting to refs over and over again it would be worthwhile to be able to pass a to-ref function to pull's context (today db-get-ref could be used as a hack).

https://github.com/vimsical/subgraph/blob/master/src/vimsical/subgraph.cljc#L408