DEP: put back veccdf/vecfunc/vec_generic_momemt in stats.distributions.

Removed erroneously in gh-3243 for 0.14.0.  These methods were always
private ones but not marked as such, and as the comments on gh-3243 show
they were used. Hence now deprecated.

(cherry picked from commit 556220bb392539b95da9e111904f5d6ebe501711)