Add collapseWithKeys to Collection

[jasonmccreary]

I had the need to collapse a collection of nested arrays while preserving their keys. Neither the flatten nor collapse methods have a flag for preserving keys. While it may not for flatten, the option may make sense for collapse given its single depth.

Consider the following collection (before/after):

[[1 => 'a'], [3 => 'c'], [2 => 'b']] // before
[1 => 'a', 3 => 'c', 2 => 'b']       // after

Of course, you may achieve the after result using mapWithKeys and a custom callback or some pipeline of collection methods. But this seems like a common enough use case for a collapseWithKeys (or better name) to exist.

[jasonmccreary]

Note, I didn't add this method to Arr or Eloquent\Collection. I don't see a use case for Eloquent\Collection, and didn't know the level of method symmetry between the Arr and Collection classes.