General Improvements for Declaration Tests and other Assertions

[Rdeisenroth]

This PRs primary focus is to provide the full functionality of the old reflect package with the new assertion api. So far this includes:

• syntactic sugar (a lot)
• checking if a method from an interface is implemented
• verifying no blacklisted statements were used
• improved error message handling (e.g. fixing cause not shown with Assertions2.call())
• add missing documentation

[Rdeisenroth]

Merging now since everyone used it anyways