Use Stan's rng_t typedef instead of hardcoding

[andrjohns]

Summary:

Helps future-proof changes to Stan's RNG by using the stan::rng_t typedef

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

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• Code: GPLv3 (https://opensource.org/licenses/GPL-3.0)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)