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.
Let's actually do 4.4.0 since 4.3.0 is going into beta soon.