Make values-to-RVTransforms available via TransformValuesOpt

[brandonwillard]

A user can add the TransformValuesOpt as an extra rewrite and have transforms applied during log-probability construction. This process starts with a user-owned and constructed instance of TransformValuesOpt, which takes a dict of values-to-transforms; however, the transforms in this dict aren't necessarily RVTransforms, since they can include the DEFAULT_TRANSFORM sentinel. In these cases, the user doesn't actually have a usable map between values and RVTransforms with which they can—say—apply their own "forward" and "backward" transforms in a manner consistent with the log-probabilities they produce with TransformValuesOpt.

To remedy this, we could perform the _default_transformed_rv checks upfront (e.g. in TransformValuesOpt.__init__) and make the results available to the user via TransformValuesOpt.values_to_transforms.