Assert on map should have keys() and values() functions

There are good assertions on individual items in maps but not a clean way to do assertions on all keys or all values in a map. So i propose the following methods be added:
```
fun <K> Assert<Map<K, *>>.keys() : Assert<Set<K>> = ...

fun <V> Assert<Map<*, V>>.values() : Assert<Collection<V>> = ...
```