Tweak automated registration for distribution-to-funsor conversion

[eb8680]

Addresses #386. Motivated by @ordabayevy's forum post about generating Funsor wrappers for custom distributions.

Currently, generic TorchDistribution-to-funsor.distribution.Distribution conversion is handled by a single pattern:

@to_funsor.register(pyro.distributions.Distribution)
def backenddist_to_funsor(backend_dist, output=None, dim_to_name=None):
    funsor_dist_class = ...
    ...

This design requires the pattern to somehow identify a Funsor counterpart funsor_dist_class to backend_dist, which it currently does somewhat hackily by looking in the global namespace. This PR tweaks this logic so that no lookup is necessary:

def backenddist_to_funsor(funsor_dist_class, backend_dist, ...):
    ...

to_funsor.register(MyDistribution)(functools.partial(backenddist_to_funsor, funsor_dist_class))

By including the final line above in the distribution wrapper generator funsor.distribution.make_dist, users should in many cases no longer need to implement to_funsor for custom distributions.

I have also deleted a superfluous conversion pattern funsor.distribution.mvndist_to_funsor whose functionality is duplicated in an eager pattern.

Tested:

• Refactor exercised by existing tests