Create single point of change for grin library imports

[yeastplume]

As described in #661, it would be very useful to use whatever cargo tools are available to make version/branch changes for grin library imports as needed. At the moment, if you're switching to a development environment or bumping versions for a build, changes need to be made in each and every crate. It would be really useful to be able to do this in one place and have it propagate through all crates.