Add support/tests for exporting functions with complex types

[andrjohns]

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

We need a very minor tweak to our expose functions routines for supporting complex types (literally 1 line). This PR makes that change and adds tests handling complex types and arrays/tuples

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
Andrew Johnson

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[codecov-commenter]

Codecov Report

Merging #861 (42b0e74) into master (a2af167) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 42b0e74 differs from pull request most recent head ccab663. Consider uploading reports for the commit ccab663 to get more accurate results

@@           Coverage Diff           @@
##           master     #861   +/-   ##
=======================================
  Coverage   88.32%   88.32%           
=======================================
  Files          12       12           
  Lines        4369     4370    +1     
=======================================
+ Hits         3859     3860    +1     
  Misses        510      510           

Files	Coverage Δ
R/utils.R	72.79% <100.00%> (+0.05%)	⬆️

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[jgabry]

Thanks!