Add Kotlin contracts to exposed Kotlin API

[awelless]

Overview

Resolves #1866. Introduced assertNull, assertNotNull and assertInstanceOf methods. Introduced contracts for assertNull, assertNotNull, assertThrows and assertDoesNotThrow methods

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Definition of Done

• [x] There are no TODOs left in the code
• [x] Method preconditions are checked and documented in the method's Javadoc
• [x] Coding conventions (e.g. for logging) have been followed
• [x] Change is covered by automated tests including corner cases, errors, and exception handling
• [x] Public API has Javadoc and @API annotations
• [x] Change is documented in the User Guide and Release Notes

[awelless]

Is there anything else that should be added/adjusted? Otherwise, I'd say the pr is in its final form

[awelless]

What do you think about the pr, @marcphilipp, @JLLeitschuh?

[awelless]

Hi. I believe the change is still relevant and the pr is ready to be reviewed.

[marcphilipp]

We'll consider it for 5.11, just need to find some time to review.