Let assertions return their parameter on the happy path

[MarkusAmshove]

(came up in #90)

Some assertions should return their parameter back to the caller, to use them as a kind of verification in unit tests.

This will also make chaining assertions possible

theString
    .shouldNotBeNull()
    .shouldNotBeEmpty()

Example:

infix fun Any?.shouldBeInstanceOf(className: Class<*>) = assertTrue("Expected $this to be an instance of $className", className.isInstance(this))

to

inline fun <reified T: Any> Any.shouldBeInstanceOf() : T = if(this::class.isInstance(T::class)) this as T else throw AssertionError("Expected $this to be an instance of ${T::class.qualifiedName}")

which has to be a separate method to not break existing code.

This would allow:

open class Base {
    fun doSomething() = println("Base")
}

class Child : Base() {
    fun different() = println("Child")
}

fun main(args: Array<String>) {
    val base : Base = Child()
    // base.different() - Doesn't compile, not of type Child
    val child = base.shouldBeInstanceOf<Child>()
    child.different()
}

Currently identified assertions:

shouldBeInstanceOf
All numerical assertions in form of: shouldBeGreaterThan
All String and CharSequence assertions
The generic should method

[MarkusAmshove]

I've just released these changes with v1.34