Minimal set of necessary steps

Working on the steps for #22 here. These are just from the top of my head:

- [x] `get` property
- [x] graph `query`
- [x] edge `source`/`target`
- [x] `inE`/`outE` (as vertex-centric queries)
- [x] `map`/`flatten`/`flatMap`
- [x] `inV`/`outV` as native steps
- [ ] `both` from gremlin (`in` + `out`) — useful for symmetric relationships (should be easy)
- [x] `fork` (△), `forkMap`

what else is needed?

Related: https://github.com/ohnosequences/puntos-y-flechicas/issues/28