introduce its as convenience feature extractor at the cost of poorer reporting

[robstoll]

Platform (all, jvm, js): all
Extension (none, kotlin 1.3): none

Code related feature

See also #790 as one of the motivations behind this.
tl;dr: with the assumption that most users weight readability of code higher than readability of the report, I decided to introduce its.

Unfortunately, Kotlin 1.4 (and most likely 1.5 as well) still suffer from, well.. how to say this, not as good type inference capabilities as I thought it would be. There are still a lot of bugs when it comes to overload resolution and it gets worse if a) lambdas are involved, b) KFunction are involved.
Hence, the current approach with KFunction will not play out that well as foreseen. its shall be a better fit for most users as I guess most users don't really care that much about reporting and prefer if the code is more readable more intuitive to write.
And I hope, depending on Kotlin's stability of IR in 1.5 that we can maybe improve the reporting in the future.

So, instead of

expect(myPerson)
  .feature({ f(it::isStudent) }) { toBe(true) }
  .feature { f(it::fullName) }
    .startsWith("rob") 

one shall be able to write:

expect(myPerson)
  .its({ isStudent }) { toBe(true) }
  .its { fullName }
    .startsWith("rob")

For the moment we will not be able to output the isStudent and fullName in the reporting and instead of feature instead (maybe, depending on how much impact it has, we will output feature 1, feature 2 etc.)

[robstoll]

I decided to include a link to the line number where the feature is defined instead of feature (feature 1 etc.). The report then looks as follows:

expected that subject: "a"        <112394081>
    ◆ ▶ its.definedIn(DummySpec.kt:36): 1        (kotlin.Int <1798371814>)
        ◾ equals: 2        (kotlin.Int <1943296756>)

Where intellij turns DummySpec.kt:36 into a clickable link.