Use rstantools - Fix Build/Export Errors

[andrjohns]

@bgoodri this PR fixes the current building errors for rstanarm (including those seen in #616), by replacing the roxygenise() call in cleanup with the generated roxygen docs and using rstantools for compiling the Stan models.

I've also expanded the CI to more combinations of OS/R versions so you can see it passing, and it's also being used as part of our R-Universe builds, which you can also see is successfully building across all platforms