Proposal: Add NonEmpty constructors to the Prelude

[Yitzchak Gale]

In continuation of Simon Jakobi's proposal for a small
incremental change to the Prelude to make it more
conducive to partial-function-free programming,

https://mail.haskell.org/pipermail/libraries/2016-December/027496.html

and after some encouragement from the community on reddit,
I propose adding the basic constructors of NonEmpty to
the Prelude: nonEmpty, (:|), and the type constructor NonEmpty.

In practice, users of NonEmpty will often still need a qualified
import of Data.List.NonEmpty. But having the constructors
available in the Prelude will make the use of NonEmpty more
common, and will make programming without partial functions
feel more natural and standard.

Note: This proposal is separate from Simon's and should not
interfere with the acceptance of Simon's. But it is related in
that it attempts to achieve the same goals.

Discussion period: Until the end of the discussion period of
Simon's proposal.

Thanks,
Yitz