Change Refabricator.between to only take fabricators

`Refabricator.between` should be changed to only take fabrics as it's arguments.
This enables to not just use static strings for `before` and `after` arguments, but any data which can be generated by a fabricator.

Resulting type:
```
type betweenOps = {before: Types.fabric, after: Types.fabric};
let between: Types.refabricator(betweenOps);
```

Also relevant: #28