`expose_stan_functions` fails for rstanarm model

Summary:

expose_stan_functions is a really useful utility and I'd like to use it to hopefully speed up likelihood calculations. Related to #117. Unfortunately it doesn't work on rstanarm objects.

Description:

The function fails because the #included files are gone by run time. The simplest solution I can think of would be to inline all the includes at package build time.

Reproducible Steps:

Make an rstanarm model

res <- rstanarm::stan_glm(x ~ y, data = data.frame(x = rnorm(10), y = rnorm(10)))

then try to expose functions

rstan::expose_stan_function(res$stanfit)

errors with output:

could not find include file: #include /pre/Columbia_copyright.stan
Error in stanc(model_code = mc, model_name = "User-defined functions",  : 
  failed to parse Stan model 'User-defined functions' due to the above error.

RStanARM Version:

2.18.2

R Version:

3.5.1

Operating System:

Ubuntu 16.04

Start at the expose_stan_function entry point and reproduce the failure with the rstanarm stan_glm example in the issue. Investigate how the included files are handled at package build time and verify that exposing functions from the resulting stanfit no longer reports a missing include file.