Reimplement split and flatten in Data.List.NonEmpty

[MatthewDaggitt]

Reasons for this change:

1. Should live in Data.List.NonEmpty.Base but because they mixed computation and proofs they could not.
2. The names weren't very descriptive of what they did. N.B. I'm open to shortening them to groupSeqs and ungroupSeqs if people would prefer.
3. They were blocking the deprecation of the inspect idiom in Attempt at deprecating inspect idiom #1630