add reverse() function for arrays and vectors

[bob-carpenter]

Summary:

reverse(x) should return the elements of x in reversed order.

Description:

This allows, for example, a decreasing vector type by reversing an increasing one.

Current Version:

v2.11.0

[bob-carpenter]

Marco --- I assigned you to this because you wrote most of the other shape and order wrangling functions. If you don't want to do it, please just unassign yourself. No rush, I gave it a target milestone of Future.

[linwangidd]

Hello. May I know if the "reverse(x)" function still not available in current version of Rstan?

[bob-carpenter]

Correct. Nobody's implemented it. It should be an easy first issue as it doesn't interact with anything else and it's a simple algorithm.

[bob-carpenter]

P.S. Almost any program using reverse outside of transformed data (where it'll only occur once) should probably be reshaped so that's not necessary.