Change Refabricator.between to only take fabricators

[woeps]

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