Allow injection of multibinding providers

E.g. we could define a `fruit::MultibindingProvider` class that allows to get a vector of the multibindings for that type, and allow injecting that as a dependency.
This will have the caveat that injection dependency loops involving a `MultibindingProvider` will only be detected at runtime.

To print the actual loop, maybe we can have an on-this-error-only, slow code path where we walk the injection graph at runtime, looking for in-progress injections, to find the loop; then we can print a more useful error message with the types involved in the loop. TBD if there's enough information available to determine the loop.