deprecate `fit_stan_survival_model`

[jburos]

In moving to the new formula-based spec for survival models (see #60), we will be deprecating the previous method of specifying survival models (thanks @julia326 for the suggestion).

We will create a new function for fitting survival models which only takes the "new" survival-model syntax. This may be called fit_model, unless there's a suggestion for a better name. We will rewrite fit_stan_survival_model to pass parameters to this new function.