new rule `reportUnusedType`

[KotlinIsland]

very useful for type testing, and detecting dead/unused types in applications

def f[T: (int, str)](a: T): ... # error: T=str is unused  reportUnusedType

f(1)

it would have to determine the different branches of types

[DetachHead]

depends on #539

[KotlinIsland]

Doesn't necessarily depend on being an application. If you enable it with tests you can use it for type test coverage