Use reformulas for formula processing functions

[aloctavodia]

From lme4 release notes:

lme4 now depends on the reformulas package for formula processing etc.; the corresponding functions (expandDoubleVerts, subbars, findbars, nobars, etc.) are still (re-)exported from the package, but throw a warning if called directly (use e.g. reformulas::findbars(...) to avoid the warning)