Vectorized overloads of `logical_` functions

[WardBrian]

This would be required for https://github.com/stan-dev/stan/issues/3300

We'd like to be able to write things like vector == real and get back and array of ints. The math library functions required:

• [ ] stan::math::logical_eq
• [ ] stan::math::logical_neq
• [ ] stan::math::logical_lt
• [ ] stan::math::logical_lte
• [ ] stan::math::logical_gt
• [ ] stan::math::logical_gte