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

- `multivariate_normal_frozen` 
- `matrix_normal_frozen` 
- `matrix_t_frozen` 
- `multinomial_frozen`
- `dirichlet_multinomial_frozen`
- `uniform_direction_frozen`
- `multivariate_hypergeom_frozen`
- `vonmises_fisher_frozen`
- `normal_inverse_gamma_frozen`