Complete covariance structures for `method_bayes()`

[danielinteractive]

Add remaining covariance structures for Bayesian imputation. Include:

• Statistical specification
• Stan parts
• Preparation of prior parameters
• Extraction of REML fit estimates
• Doc updates
• Tests

To do:

• ar1h
• cs
• csh
• ad
• adh
• toep
• toeph
• NEWS update