Turn MapAssertions into MapLikeAssertions

[robstoll]

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

Code related feature

We turned IterableAssertions into IterableLikeAssertions, allowing them to be used for types which can be converted to Iterable.
Similarly it makes sense to do the same with MapAssertions, turn them into MapLikeAssertions.
This way we can for instance provide the assertion functions to Scala's Map types with ease.