findbars and friends are moving to the reformulas package

[bbolker]

findbars and other formula-processing machinery has been moved to the reformulas package. In the current development version of lme4, these functions are no longer exported from lme4. I plan to partially revert this change so that lme4::findbars issues a deprecation warning ("please import this function from reformulas instead") rather than throwing an error, but in any case it would be good to move rstanarm to import these functions from reformulas rather than lme4.

I can submit a pull request if you'd like.