Vectorized overloads of `logical_` functions

[WardBrian]

This would be required for stan-dev/stan#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