Add rule to replace function body with single expression by an expression body

[paul-dingemans]

Kotlin Coding Conventions:

Prefer using an expression body for functions with the body consisting of a single expression.

fun foo(): Int {     // bad
    return 1
}

fun foo() = 1        // good

Android Kotlin styleguide:

When a function contains only a single expression it can be represented as an [expression function]

override fun toString(): String {
    return "Hey"
}

override fun toString(): String = "Hey"