cpp_object_initializer failing in downstream packages

Summary:

Downstream packages can't use rstan without dependsing on Rcpp, which would pollute the user's namespace

Description:

This was previously discussed in #176.

The Rcpp developers have discussed improved solutions in the last week or so on https://github.com/RcppCore/Rcpp/issues/168; If I understand correctly, it might be solvable by adding importFrom(Rcpp, evalCpp) to the NAMESPACE.

Reproducible Steps:

(via a mini package I created to demonstrate the issue)

devtools::install_github("davharris/fail")
fail::test()

Current Output:

Error in .Object$initialize(...) : 
  could not find function "cpp_object_initializer"
failed to create the sampler; sampling not done

Expected Output:

an 8-schools stanfit

RStan Version:

2.12.1

R Version:

R version 3.3.2 (2016-10-31)

Operating System:

10.12.1

Start by reproducing the failure with the commands in the issue and inspect the linked mini package's R/hello.R entry point. Compare the package namespace behavior around Rcpp and cpp_object_initializer, then verify completion by confirming downstream packages work without depending on Rcpp and the 8-schools example produces a stanfit.