(tx) Support (lookup-ref a v) and (current-tx) in TermBuilder

[ncalexan]

It's not easy to arrange, but it hurts to not be able to use lookup refs and especially transaction functions like (current-tx) (after #665) in the term and entity builders. The fundamental issue is that the meaning of a chunk of EDN given to to the transactor is context sensitive: it depends on the schema (in particular, to determine if entids or idents should be coerced to TypedValue::Ref). TermBuilder simplifies the space of value inputs, and therefore doesn't have to deal with all of the context sensitive inputs... but in turn it pays by not providing access to all of the input space.

[ncalexan]

This landed as part of #778.