`stats._multivariate`: generic shape-types for certain `*_frozen` types

in order of populatity (according to github code search):

- [x] `multivariate_normal_frozen`
- [x] `multinomial_frozen`
- [ ] `multivariate_hypergeom_frozen`
- [ ] `normal_inverse_gamma_frozen`
- [ ] `vonmises_fisher_frozen`
- [ ] `uniform_direction_frozen`
- [ ] `dirichlet_multinomial_frozen`