Introduce TestResult

[jcornaz]

I said in #51 that:

• forAll is good because the compile will enforce the assertion (in the form of a predicate)
• checkForAll is good because using an assertion library such as Kluent or similar gives better message in case of falsification

But it turns out we could have the best of both worlds if the property function returned an TestResult (like the Expectation that must be returned when using elm-test's fuzzer).

With that we could have both: assertion enforced by the compiler and good messages in case of falsification.

So this issue is about introducing the type TestResult and require property tests written with forAny to always return a TestResult.

A minimal starting API could be:

sealed class TestResult {
  object Success : TestResult()
  data class Failure(message: String) : TestResult()
  object Skip : TestResult()
}

fun Boolean?.alwaysTrue(): TestResult = TODO()
fun Boolean?.neverTrue(): TestResult = TODO()
fun Boolean?.alwaysFalse(): TestResult = TODO()
fun Boolean?.neverFalse(): TestResult = TODO()
infix fun <T> T.alwaysEquals(exptected: T): TestResult = TODO()
infix fun <T> T.neverEquals(exptected: T): TestResult = TODO()