Implementation for assertArrayPart #1371

I probably will not have time to open this PR later this weekend so I decided to do it now, just in case the feature discussed at #1371 get's approved :D

In case it gets approved but with a different assertion name I'll be happy to update the PR, just ping me.

Cheers.
Should we offer `==` and `===`  like `TraversableContains`? Might even be nice to use the comparator subframework like `IsEqual` does since it offers some more options and some improvements for recursive data structures and diffing.