Add the refmap construct

[clyfe]

The refmap construct is similar to refset, but resolves to a map from
keys to resolved refs, rather than a set of refs; it is useful when the
keys are meaningful to the logic beyond just being a slot in the config
map, and are required in further logic.

Resolves #63

[weavejester]

Please ensure the code and commit message adheres to the contributing guidelines.

[weavejester]

Thanks. Can you separate out the introduction of the additional protocol method into its own PR? I'm not sure about #ig/refmap, but I'm more willing to accept an extra protocol method to allow that functionality to be added by users. I think I'd also want to call the extra method ref-resolve rather than resolve-ref.

[clyfe]

Rebased on feature/65-reflike-resolve

[clyfe]

@weavejester Can you approve or reject this one? Or are you still uncertain?

[weavejester]

My thought is that this might be a little too specialized. I'll add the ref-resolve method to the next alpha so this functionality can be added manually, but I don't currently think it should be default behaviour. However, I'll keep this in mind as a possible future addition.

[clyfe]

Standalone implementation here: https://github.com/tape-framework/refmap