Don't error when fitting a model using reloaded CmdStanModel with compiled model methods

[jgabry]

Submission Checklist

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

Summary

Closes #1157

CmdStanModel objects created using compile_model_methods = TRUE that are
then saved and reloaded no longer error in model fitting methods. Model methods
are recompiled lazily if needed.

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):
Jonah Gabry

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

❌ Patch coverage is 81.25000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.64%. Comparing base (47596d3) to head (ed3dd1f).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
R/utils.R	78.57%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1158      +/-   ##
==========================================
+ Coverage   90.57%   90.64%   +0.07%     
==========================================
  Files          14       14              
  Lines        5921     5934      +13     
==========================================
+ Hits         5363     5379      +16     
+ Misses        558      555       -3     

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