Inconsistent `arena_matrix`/`varmat` interaction between `=` and `+=`

Opening this one as an issue since I'm not sure what the correct/intentional behaviour should be!

The arena_matrix <-> var_value<Eigen::MatrixXd> interaction currently handles inplace writes differently to assignment.

Where modifying an arena_matrix after creating a var_value will also update the value of the var_value if an inplace write is used, but not if an assignment is used:

#include <stan/math/rev.hpp>
#include <iostream>

int main() {
  Eigen::MatrixXd m = Eigen::MatrixXd::Constant(1, 1, 1);

  stan::arena_t<Eigen::MatrixXd> a(m);
  stan::math::var_value<Eigen::MatrixXd> v(a);  // shares a's memory
  std::cout << v.val() << "\n";

  a += m;  // writes in place, v sees the new values
  std::cout << v.val() << "\n";

  a = a + m;  // rebinds a, v keeps the old memory
  std::cout << v.val() << std::endl;
}

Returns:

$ ./arena 
1
2
2

Start with the provided reproducer and inspect the interaction between arena_tEigen::MatrixXd and var_valueEigen::MatrixXd. Determine the intended aliasing behavior for += versus =, then define a consistent expected result and tests for both operations.