Support more consequences of non-empty collections

[janopae]

Since #94, this extension supports the scenario of checking a Collections's emptiness and then using first() and last() on that collection without worrying about the return type being false. This is great, however, a Collection being empty has more effects than this, including:

• toArray() returning a non-empty-array
• getKeys() returning a non-empty-list
• getValues() returning a non-empty-list
• code in loops iterating over a non-empty array will be executed at least once

Also, a Collection being empty could be asserted by

• checking count() returns a value other than 0
• calling add() at least once
• calling set() at least once
• calling a custom method that was annotated with @psalm-assert !$collection->isEmpty() with the collection
• checking contains() returns true
• checking current() doesn't return false
• checking key() doesn't return null
• checking next() doesn't return false
• the Collection being created using the map() method of another non-empty Collection
• checking indexOf() doesn't return false

[orklah]

Seems cool! Can you propose a PR based on this?