Constructor functions for Show to use receiver function

[pakoito]

From gitter, current encoding

val catShow = Show<Cat> {
        val name = it.name
        val age = it.age
        val color = it.color
        "$name is a $age year old $color cat"
    }

with proposed encoding

val catShow = Show<Cat> {
        "$name is a $age year old $color cat"
    }

[pakoito]

Eq cannot have receivers for two parameters, silly me :P

[pakoito]

Closed by #835 . Thanks @ersin-ertan