Proposal: Add singleton function to Data.List module

[Herbert Valerio Riedel]

> - `(:[])`: Subjectively ugly.

I consider "subjectively ugly" to be a non-technical and thus really
weak argument to dismiss the list-idiomatic ninja-robot-operator (:[])
which also happens to be shorter than the proposed alias for it. I for
one don't see a significant benefit for adding a redundant synonym to
`Data.List` and are thus -1 on this.

> singleton :: a -> [a]
> singleton x = [x]